I don't think that, for most people, it's as simple as someone being a homosexual. What is it to be a homosexual? Is someone who has gay sex once a homosexual? Is someone who is attracted to a man once a homosexual? And if not, then why not? And if so, then where do we draw these lines?
The "one act" policy is a lot like the "one drop" policy.
I think that sexuality in general has suffered from a surfeit of words, as has beauty. We use these words to define, to specify, to make a phenomenon a static entity, to make an idea compartmentalized, and we ignore whatever lies on or travels across the edges. The word creates an assumption of a certain uniformity.
There are people who prefer only men, and there are people who prefer only women, and there are many, many, many who prefer both to different degrees, or who would say "it's not about the gender, it's about the person."
So often, the assignment of a label provides a site of inquiry--"If I am this, then what on earth is that totally different thing over there? Can it stop doing that and be more like me?" rather than moving to a person and saying "here I am--now how are you?"
Our assumption about language is that it is static, so once a word falls on you it never leaves--much as we assume that psychological conditions are static, and once they fall on you they never leave.
I don't think the way we use language is exactly just, and I think the question of "genetic or choice" is a little bit silly, and largely engineered by our assumptions about language and psychology. We don't make this big deal about our preferences for ice cream. It's a bit boorish to argue about taste.