*Definitely* an old wives tale, dating all the way back to the time of Aristotle (335 BC). This has no scientific basis.
As you probably know, sex determination in humans (and most vetebrates) is determined by the presence of the Y chromosome. In humans, sperm carry 22 autosomes and a Y or 22 autosomes and an X. If they carry X, the offspring is female, if Y, the offspring is male (since the mother has only X to offer). When sperm form, they have a 50-50 chance of carrying Y. There is no tipping this balance in temrs of making "more Y or more X." One could argue that perhaps X-bearing sperm "die off" if there is frequent intercourse, leaving only the Y sperm available, but that doesn't hold any water scientifically. Aristotle and friends didn't know this, but invoked many explanations for sex determination, including how passionate the man was. In later years, many men advocated the "more sex, more sons" theory for socio-cultural-religious reasons that have no basis in science.

As far as I know, the ratio of sperm carrying the x or y chromosome doesn't change with frequency of sexual activity. However, there are some basic differences between x and y carrying sperm that do have an influence on sexual selection.The x carrying sperm is generally more resistant to the basic environment of the female uterus, while the y carring sperm is a faster swimmer. There have been experiments to show that if the pH of the uterus is changed to more acidic, then the likelihood of a boy child is increased, while if the pH is left more basic, girls are the result. I am not sure how the pH of the fluid in the uterus would change with frequency of sex, but it would seem likely, as the fluid associated with the sperm is more acidic, that the pH balance of the uterus would be tipped in favor of boys. The differences, of course, are relatively small.

My specialty is definitely NOT sex determination in human development but I did a little searching on the web. According to one article on the Scientific American web site (http://www.sciam.com/1998/0798issue/0798scicit3.html), XY zygotes are conceived 25% more often than XX zygotes. Boy babies are only slightly (1.2%) more likely than girl babies, however, because male fetuses are less likely to be carried to term than female ones. The article goes on to discuss recent research surrounding a dubious statistical relationship between higher pollution exposure and lower male birth rates. (Exactly the kind of science that will guarantee you a huge press release.) However, I'm including the article in my e-mail because in the last paragraph it discusses why male babies might be more likely to be born to couples that have sex more often. (It is just a theory, however.) According to the article, it is not because males produce more Y sperm but because frequent sex increases the chance of conception early in the menstrual cycle, which may make a male baby more likely. It is not clear from the article if this is because (a) the egg is more susceptible to Y sperm than X sperm early in the menstrual cycle or (b) the chances of carrying a male baby to term increase during early conception due to a more favorable womb environment.

Anyway, here's the article:

WHERE HAVE ALL THE BOYS GONE?

The mysterious decline in male births

Despite their macho swagger, males are the more fragile sex of the human species. Male fetuses are less likely than females to come to term: although 125 males are conceived for every 100 females, only about 105 boys are born for every 100 girls. In the first half of this century, improvements in prenatal care reduced the number of miscarriages and stillbirths and hence increased the proportion of baby boys in most industrial countries. But since 1970 the trend has reversed: in the U.S., Canada and several European countries, the percentage of male births has slowly and mysteriously declined.

So far the decrease has not been alarmingly large. In the U.S. in 1970, 51.3 percent of all newborns were boys; by 1990, this figure had slipped to 51.2 percent. But in Canada the decline has been more than twice as great, and similar long-term drops have been reported in the Netherlands and Scandinavia. The U.S. and Canadian data were compiled by Bruce B. Allan, an obstetrician-gynecologist at Foothills Hospital in Calgary, Alberta. Allan claims the widespread nature of the decline suggests that it is more than a statistical fluctuation. "We can't deny that the percentage of boys is falling," Allan says. "But the question is, Why?"

Demographic factors may be playing a role. Different races have slightly different birth ratios: blacks tend to have fewer boys than whites, whereas Asians have fewer girls. (These differences have been observed worldwide.)
The parents' ages may also influence the gender of their offspring; studies have shown that older fathers sire fewer sons than young dads. But Allan found that demographic changes in the Canadian population between 1970 and 1990 could not account for the decline in the percentage of baby boys there.

Some researchers believe pollution may be the culprit. A recent article in the Journal of the American Medical Association notes that high exposures to
certain pesticides may disrupt a father's ability to produce sperm cells with Y chromosomes-the gametes that beget boys. Other toxins may interfere with prenatal development, causing a disproportionate number of miscarriages among the frailer male embryos. (XY embryos require hormonal stimulation to produce masculine genitalia, which may make the unborn males more vulnerable to hazardous chemicals.)

Perhaps the most striking example of a lopsided birth ratio occurred in Seveso, Italy, where a chemical plant explosion in 1976 released a cloud of dioxin into the atmosphere. Of the 74 children born to the most highly exposed adults from 1977 to 1984, only 35 percent were boys. And the nine sets of parents with the highest levels of dioxin in their blood had no boys at all.

Devra Lee Davis, a program director at the World Resources Institute and one of the authors of the JAMA article, argues that the declining male birth ratio should be viewed as a "sentinel health event"-a possible indicator of environmental hazards that are difficult to detect by other means. But other researchers say the link between pollution and birth ratios is not so clear.
Fiona Williams, an epidemiologist at the University of Dundee in Scotland, found no correlation between birth ratios and levels of air pollution in 24 Scottish localities. Although very high levels of certain pollutants may reduce the percentage of baby boys, she concludes, one cannot assume that lower

There are several interesting questions within this one. Some are physiological, some evolutionary. There's actually reason to believe that the bodies of animals can adjust sex ratio in offspring. I'm a behavioral ecologist who studies animals, so I'm not sure whether the evidence is good in humans, but here's the general scoop. First, the number of X and Y chromosomes per male does not change. All cells in the body that have nuclei have the same 23 pair of chromosomes. In a male, one "pair" is actually an X and a Y. When sperm cells are formed, each male stem cell produces 4 sperm. 2 get a Y (plus 22 other chromosomes), 2 get an X (+22). The Y-bearing sperm have less mass because the Y chromosome is smaller than the X. Y-bearing sperm tend to travel faster, but do not remain viable as long as X-bearing sperm. These physical differences have led to some techniques for "sorting" sperm by type (Discover, Jan., pg 57). So physiologically, there's a difference, which would at least allow a difference in "handling" by the body.

Now let's look at the evolutionary argument. Fitness means leaving proportionately more offspring in the next generation. So is it better to have sons or daughters? Let's look at some deer. We'll have to simplify things, but it will still get to be a complicated picture. I'll talk about what a deer would "want" as a shorthand for "this strategy will tend to benefit the individual in an evolutionary sense, even though it would not make a concious choice".

Say there are 10 females and 10 males. Only one male gets to breed. He keeps the other 9 males away from fertile females. Now the AVERAGE fitness of all individuals is the same. Each female has one offspring, so her fitness = 1. 10 offspring were produced, and there are 10 males, so the average fitness for males = 1. If we were only interested in averages, sons and daughters would have the same payoff. However, the variation in payoff for males is obviously much greater. A son may leave 10 offspring per year, but is much more likely to leave 0. A daughter has a low, but almost guaranteed, payoff of 1/year. With just that info, it seems like the best option might be to go for a 50-50 sex ratio: make the secure investment in females, and also make some high risk/potentially high payoff investments in males.

Now things get more complicated (and interesting). What if a parent could increase the chance that their son would be a "winner" son instead of a "loser" son? Then are sons a better bet? If a male is dominant due to genes alone, both parents might want a son if the father is the dominant deer. But if a parent produces too many males, he or she loses anyway; only 1 or 2 of the sons are going to get to breed and there will be fewer females for the "winner" sons to breed with. Note that the value of males goes down as the proportion of males goes up. For daughters, the payoff is still 1/yr. Also remember that a female's genes will make up half the offspring's genes. Also, the mother's health may be a big determinant of whether the son grows up to be a winner,
whatever his genes are. So a dominant male may still want to have a daughter, especially with a lower quality female. The mother would want this too.

The cost of raising a male is also higher for the mother (though not for the father, since he provides no parental care), so a female may want to leave fewer sons than a father would. A "loser" male who happens to sneak a rare copulation may want to leave a daughter since his sons would be likely to leave few offspring. The female would agree.

Some animal studies indicate that indeed, better quality females have more males. This is true even if the father is the same. That is, if females are bred by the same male, females that are put on poor diets will produce more females.

So I still can't answer your original question for humans, but perhaps studies have actually been done on it. Getting accurate data would seem to be a real challenge, though.

An ob/gyn would probably be best suited to answer this question. I have heard in a biology course that male sperm swim faster than female sperm therefore they wear out their energy reserves quicker and die. Female sperm swim slower, but live longer. I have heard that this information has been used to some success by people trying to have one gender or another when they are trying to get pregnant. You would have to figure out the exact day the female ovulates on. If you want a boy you wait 1-2 following ovulation to have sex and then hope the egg gets far enough down in the fallopian tubes that the male sperm will reach it before the slow female sperm. If you want a girl, have sex about a day before ovulation so that the female sperm will be the only ones still alive by the time the egg gets into the fallopian tubes where the sperm are. Here is a website that sheds some light on the info I heard a long time ago in a biology class.
NOTE: my timing with ovulation is apparently off, read the page:
http://babycenter.com/refcap/2915.html

Specifically addressing your question is the first page of the following web page (which basically says it is an old wives' tale.
http://www.ihr.com/westcoast/articles/choose.html

Check out additional info at the following web sites:
http://www.selnas.com/en/n4/selnas-aventure.html
http://www.selnas.com/en/n4/selnas-methodes.html#top
http://www.selnas.com/en/n4/index.html