What I am saying is that same-sex marriage and heterosexual marriage are two different relationships (a point you conceded).Yeah, sure. But only in the most trivial sense imaginible: Yes, I concede that homosexual marriage is different from heterosexual marriage in that the former is not -- at present -- likely to result in offspring. Well, to be precise: Not the simultaneous traditionally biological offspring of both partners in the marriage. (Though I can see at least one way to get that, too.)
What I didn't, and won't, concede is that this difference is in any way whatsoever significant. Marriage is one thing, procreation another, and conflating them is just simply muddled thinking.
HTH!