I don't know why you guys went from marriage to talking about HIV/AIDS
One could almost propose, similarly, to Godwin's law that in discussions regarding homosexuality the chance of HIV being blamed on gays (as proof that homosexuality is not harmful) is near unity. One could also presuppose that the chance of mention of (a) rape, (b) incest, (c) pedophilia, (d) bestiality or (e) destruction of humanity due to lack of breeding being mentioned as a comparison to adult consensual homosexual relationships IS unity. For discussions specific to legal recognition of homosexual unions the chance of mention of the five cases above with addition of legalization of polygamous unions or of legalization of human-non human unions (cf. Rick 'Man on Dog' Santorum...Google is NSFW in the case of Santorum) is so near unity as to be insignificantly different.