The 'boy' sperm swims faster, but does not live in the uterus for long. 'Girl' sperm moves slower, but stays alive longer. So if your egg is ready to be fertilized when sperms are introduced, the boy sperms are more likely to get to it first. If the egg comes late to the party, only the girl sperms are left... something like that... That's the advice a friend of mine (in New York) got from a specialist when she realized she doesn't want to shell out USD 10K to artificially impregnate herself to get a boy. I have heard of women getting it done in Asia before, wouldn't be surprised if it's popular here too. I'd imagine it would be legal as it is like artificial insemination, except that they sort the sperms accordingly before inseminating?