Several contracts ago I worked with a man who had 3 daughters. They'd all be about high school age now, and if they're anything like their baby pictures they're all dolls.
But being the bastard I am, I couldn't leave him alone about this. At the time I had read of various ways to affect the sex of your baby, and...well, follow along for a minute.
Number one, for a vast oversimplification, boy and girl sperm are different - the boys are sprinters and the girls are long distance runners. So if they don't have to go too far, the boys win. This invites speculation that the better hung among us are more likely to have boys, which does seem appropriate in some cosmic way. Strike one for Tommy.
Number two, the boy and girl sperm react differently to the pH (acidity) of the vagina. This pH changes at orgasm in such a way as to favor the boy sperm. Again, there's some cosmic justice here. So guess what, pal? - you're a lousy lay too. Strike two.
I'd never cut Tommy any slack (and don't go thinking I didn't get ragged right back), but in fact as noted this is a gross oversimplification. The woman's own anatomy certainly plays a role in both situations - vaginas are not all of the same length, and the sperm still has a way to go yet (although I suppose it's possible in principle to fertilize an egg that has managed to get to the vagina, I've never heard of a case of one that implanted. For more info on strange pregnancies, check out this on Beth's site here). Of course where Mama is in her menstrual cycle matters and lots more besides.
On the man's side, it certainly depends on the velocity of the sperm, which isn't exactly consistent. The relative positions when sperm are released matters too - doing it mostly on the downstroke would give Junior a head start over Sis.
No, I'm not a medical professional - I'm just following up on the implications of what I recall from back when I could devil Tommy locally. Heaven knows how much more we've learned since then, and I would not recommend attempting to learn science from bull sessions. Maybe Medpundit, MedRants or the Bloviator will chime in with the latest poop.
For those of you seeking to choose the sex of your baby, you might find some useful info here. Or you can just check out old wives' tales here.
As for the orgasm part, well, I'm here in St. Louis. Here in the Show Me State, we don't just do it, we study it - this is the home of Masters and Johnson. (For the kinkier stuff, head a couple hundred miles east to Bloomington, IN, the home of the Hoosier Review and the Kinsey Institute. We'll bring a feather to bed: they take the whole chicken).
Oh, what I could share! There's the Venus Butterfly, and Altoids...
But other than that, I'm not telling.