My friend is trying to get pregnant and truly believes that she will not have a boy unless she has an orgasm. Here is her science:
I would love to see this theory debunked. 
Eh, sex selection aside, she’s confused about the role of female orgasm in uterine acidity, which isn’t so simple.
The PH level is affected by whatever fluid is present in the vagina during the orgasmic cervical dip, which sucks fluid up into the uterus.
In the case of a female orgasm that occurs before the man’s orgasm, acidic vaginal fluid is taken up into the uterus, making the environment less fertile.
If the woman comes after the man, the basic fluid excreted by the man’s prostate gland (combined with semen) is taken up into the uterus. This makes for a more fertile situation because (apart from giving the spunk a leg up on its journey) the acidity of the uterus is mitigated.
Yes, the female orgasm effects fertility: If it occurs before male ejaculation, it reduces it-- if it occurs simultaneously, or just after, it increases it.
I’ve never heard anyone suggest that “female” sperm are more tolerant of an acidic environment before. I’d be very surprised if it turned out to be true-- if only because, if it were, women would obviously outnumber men by a huge margin.
(On preview, I think “orgasmic cervical dip” would make a great salad dressing.)
Former sex-ed teacher checking in:
The female orgasm should increase chances of pregnancy: leading up to and during orgasm, movements and contractions of the uterus, and the changing shape of the vagina to create an expanded and lengthened chamber and the orgasmic platform make it easier for sperm to enter the uterus.
I’ve never heard of acidity killing types of sperm based on the chromosomes present within the sperm, and I seriously doubt that it happens.
Acidity is in general bad for sperm regardless of the chromosomes within. That’s why the male secretes alkaline fluids from the prostate and bulbourethral glands, to raise the pH of the vagina from about 3.5 or 4 up to about 6 so the sperm can survive.
Oh! I stand corrected about the timing of the female orgasm… didn’t see your post Larry Mudd, and I hadn’t thought of that. 
You’d expect this to be confirmed statistically, depending on whether a culture values the achievement of female orgasm during intercourse. A prudish culture that does not believe women should take pleasure from the reproductive act would produce proportionally more female offspring than a culture that believes both partners should enjoy sex. In the US, for example, the liberalization in attitudes about sex that occurred in the 60’s meant that more women could discuss their desire to reach orgasm, and more couples could discuss how they might go about ensuring that both partners enjoy sex. This isn’t to say that no American women had orgasms before 1965, of course, but the group that didn’t tend to have them would have become smaller, and you’d expect that birth ratios would have moved in favor of male offspring.
In other cultures, the male/female ratio tends to be roughly the same unless male children are considered advantageous, and then the reason for there being more males than females is quite darker than women having orgasms. Basically, you’d expect sexually liberalized cultures to have more males than sexually oppressive ones, and generally this is not observed.
It is possible, though, to mechanically separate X and Y sperm outside the body, then implant only the desired sperm. This provides a certain degree of control over the gender of the offspring. Of course, this raises substantial ethical concerns and is not, AFAIK, done in humans. It is, however, used in agriculture where artificial insemination is common and one gender of animal is preferred over another, such as with chickens and dairy cows.
Human sex selection via flow cytometry has been commercially available for a couple of years.
It’s important, however, (at least for the purpose of this thread) to note that flow cytometric separation is dependant on X-sperms’ slight size advantage over Y sperm – not on manipulating PH levels to “weed out” males.