I've heard this
concept argued before, when a rant by one of the more
homophobic members of the
University I attended found its way into the
local student rag, called
Gair Rhydd, only to be
shot down in flames (and fittingly in my
opinion).
To anybody with even a GCSE knowledge of biology knows, genetic structure is determined at the point of conception (barring a few possible mutations), so if you were gay then you would have a very high likelhood of not having kids, and therefore this alleged "gay gene" wouldn't last long.
On the point that Saige made about identical twins, I have a pair of identical twins as friends. One is straight and engaged, whilst the other is a lesbian. To my mind this pretty much rules out this argument.
In response to later posts: Ok so the recessive gene idea slipped my mind, but this still doesn't explain the twins differing sexualities. Although I accept that sexual orientation is very much a grey area, either one or the other of the twins is fooling themselves, or the gay gene idea is straight out of the window