So for as long as there have been gay people, man has wondered why that is. More recently people have been wondering what causes gayness, a gene, the environment, eating too many popsicles? Well according to the most recent dear science podcast it's probably a gene (at least in men). Go listen, it's short and sweet, but sums up the various possibilities and what the current thinking is on the topic.
One thing that was touched on briefly was the possibility of "curing" gayness, but was dismissed as impossible for the foreseeable future. This is the sort of situation where science fiction becomes handy, it lets people discuss these sorts of issues long before they become possible. My opinion is that some people probably would want their children cured (once possible), but that the risks would probably be too much for most parents. It would probably be ethical (since altering embryos is already fine in e.g. IVF), but for parents that want to have a child I doubt many are going to want to abort or have a good risk of spontaneous abortion because of a raised chance of having a gay child (note that it could only be a raised chance, or else we would have already have found it, this is my problem with most Gattica style genetic dystopias: genetics is mostly chance). However this would depend on the culture: if the trends continue and people become more and more accepting of gays then I would think that my scenario is the most likely (especially since there might still be the chance for grandchildren as more gay couples adopt), however if there is some big backlash then they might view gayness as on par with real genetic diseases.
And I'm sorry lesbians, no one knows why you are gay. Or why women's sexuality seems to be more flexible than male sexuality (which is a topic for another post someday).