They also say if you hang a ring on a thread and dangle it above a pregnant womans belly you can predict the sex....
I did it for all 7 of my kids...and it was spot on! Coindidence maybe but that is a good coincidence

Boy sperm are much more fragile than the girl sperm, the heat kills them off quite quickly.
That is why if you are trying for a boy DTD right on ovulation as the boy sperm swim faster but die quicker.
So I agree with Jellybean tight lycra and long hot sweaty rides would do that.