You've given this a lot of thought. Just to clarify a couple points, most gays and scientists who study such things don't think it has been shown that sexual orientation is strictly genetically determined. Rather the best evidence is that it's determined by a combination of factors probably including some genetic ones. What's more clear is that people generally don't choose their orientation, that it's due to factors beyond our control. I don't recall choosing to be straight. I doubt very much that your nephew chose to be gay. (We could wonder why anyone would in such a hostile climate.)
Whether sexual orientation is chosen is thought by many to determine its moral status, but that isn't the case. The Catholic and Mormon churches, for example, officially accept that sexual orientation may not be chosen, but they still oppose homosexual behavior.
Some analogies between race and sexual orientation may be dubious, but there are some striking parallels in the social developments and the legal arguments.
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