The human genome doesn't work exactly like that. Genes for homosexuality, if they exist (or predisposition to homosexuality, w/e) are part of our collective genetic heritage. A great deal of the information within our individual (And thus collective, as we all come from a single organism) DNA code is simply not expressed. There's too much information -- DNA appears to adapt all of it's information to it's environment and create a being that fits that environment.
It is, surely possible to EVENTUALLY extinguish "the gay gene" from the human race, probably, but is that really a direction we want to go? I mean, why would we even consider it, is is that gross? Fuck, we have an overpopulation issue on Earth anyways.
Basically, even if no homo ever reproduces again, the human genetic code still has homosexual's written into it, and they will still exist, unless a genetic fluke happens that removes it.
Much of human behavior, including homosexuality, may have simply been a fluke -- random, freak, byproducts of evolution that aren't harmful enough to cause serious ripples in the gene pool.
But, again, in the future, who knows. In the book
The Forever War by Joe Haldemaan, which deals with the problems of war in space and relativity (Earth changes drastically over many years every time he returns from tour, because of the relativity), one of the final chapters, everyone on earth is a homosexual and look at hetero's like the fucking people who get dropped off at the end of Close Encounters of the Third Kind.
Great fucking book by the way.
http://www.amazon.com/The-Forever-Wa.../dp/0312536631