I do know someone who was raised by lesbians but is heterosexual (straight) herself, so I agree with Rhedman on that point. Then again I could see Baba Ras Marcus' point as well. I think ones can question whether it's right for gay couples to adopt kids without suggesting that they are all a bunch of child molesters (not saying anyone here did, but for the record child molesters come in all shapes, sizes, genders and sexual orientations and most child molesters are heterosexual, since most people are heterosexual). I do think it is important for children to have role models of both genders to give a balanced view of life. But I question the whole entire "nuclear family" structure that is promoted as the ideal in the west, regardless of parent/guardians' sexual preferences. It's a cliche that "it takes a village to raise a child" but some cliches are true and I think that one is.
I don't have anything against homo/bisexuals as such. I've come into contact with a few in work and life in general and have been able to get along fine with them. I have to say that when my grandmother was in hospital in London suffering from a stroke she got better care from one flamboyantly gay filipino nurse than she did from some of her fellow west indian "sisters" who worked there.
As for whether or not it's ever "natural", I don't know. The article three-sixty linked showed some examples of homosexual behaviour, or what could be interpreted as homosexual behaviour, in the animal world. Maybe looking at indigenous cultures that have remained relatively untampered with by the modern world might give us a clue as to whether it ever "naturally" occurs - but then, how many such societies still exist? very few. I do think some people identify as "gay" or "lesbian" due to environmental factors, upbringing etc., but maybe there are others who are born with that tendency, just like there are occasionally people born with six fingers or other "abnormal" features. Who knows? Not I.