There is evidence that a part of it is genetic. I can't recall any where they compared adopted and non-adopted siblings to see if it was tied to upbringing, but I've heard that there are also some links to early-life lifestyle.
well, it is genetic, but insomuch as you will see it in the genes, but it is in everyones' genes. A lesbian getting pregnant is no more or less liking to produce homosexual offspring than a heterosexual woman.
I've heard studies saying, also, that it is something that is solidified by the age of three, during which time our sexuality and gender identity are formed (which I guess is possible.) But you can't condition or train your kids not to be gay or identify with a different gender. But those studies are outliers and I would reject them as the majority of geneticists and doctors say it is from birth.
The point is, if you are expecting homosexuality to die out so that proper, decent heterosexuals can go on without them, you might want to consider that homosexuality has been around since the beginning and has yet to die off.