Counterargument against Homosexual as Innate
One of the major arguments for the acceptance of homosexuality is the fact that a person cannot choose to be homosexuals. — NukeyFox
Choice is really only an issue due to religious objections, which are undermined by the acceptance that people are born that way and that homosexuality is thus a creation of divine will. For those who consider this state a perversion, the involvement of their deity in proliferating it causes a problem. Hence the denial of the natural state and insistence on the primacy of choice.
For those who are not religious and focus purely on the ethical issue, none of this matters because, as has been pointed out, the question is only whether or not homosexuality causes harm (and here you can focus on actions rather than the state). So, you can render religious objections problematic to the extent that you can demonstrate homosexuality is innate and you can refute ethical objections by showing it causes no harm. In other words, it's important to disentangle the religious from the ethical and the state from the action in order to deal with objections to homosexuality, but once you've done this, it's straightforward enough.