www.betterhumans.com/News/news.aspx?articleID=2004-10-13-4

New study out of Italy suggests that the genetic basis for male homosexuality is carried on the x chromosome and is passed through female ancestors.  They found that the trade off for this is significantly increased fertility for the female relatives of gay men.  Which more than compinsates for the gay men dropping out of the reproductive race.

Of course this says nothing about lesbianism though.