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There have been very few studies on separated identical twins and homosexuality. There have been a few more non-separated identical twins studies on the subject.
If the tendency for homosexual behavior was purely genetic, then, when one twin exhibits the behavior, the other twin should (in most, if not all cases) also exhibit the behavior. Conversely, when one twin exhibits the tendency for heterosexual behavior, the other twin should also do so.
The studies show, however, that when one identical twin is homosexual, the other twin is usually not! In fact, the tendency for homosexuality in twins is about the same as in the general population. The conclusion seems to indicate that this is a case of "nurture, not nature."
Is this a simplistic overview of the data? Yes. But it is hard to explain away the results without resorting to the environment of upbringing and social interactions of the participants.