A new Canadian study to be published today indicates that one in seven gay men are born that way due to the mother having had sons prior to giving birth to them. The womb "remembers" whether there were sons born previously and the more sons a woman has, the more likely the next son will be gay.
Here is the article for anyone interested:
Study links older brothers, homosexuality
Remember, this study only claims to explain one in seven homosexual males. There are still six of seven that don't fit this particular profile and it has no bearing on lesbians at all.
I think what I found the most interesting about the article was this section, towards the end:
Meanwhile, some have also argued that people are more likely to be tolerant of homosexuals if there are biological and not environmental explanations for sexual orientation.
Now, why do you suppose they threw that in there???