Yeah, I see your point-- this is something that I've been concerned about myself, actually, in a more overarching way: that our emphasis on biology could be misinterpreted and misused to mean women = baby machines and LGB = unnatural. Because the real physical differences between men and women have all too often been exploited for that purpose.

So that's a much more all-encompassing issue to be addressed. To restrict myself to the anti-same-sex-marriage arguments that you've predicted, here are some of my responses:

1. Yes, this is why a gay man, for example, cannot marry a woman (she won't ever turn into, or be a substitute for, a man), and therefore why same-sex marriage is a necessity.
2. Marriage being a human creation, it is what we say it is. In many U.S. states, marriage was once defined as being between a man and woman of the same race. Then the definition was changed to allow mixed-race marriage. This did not somehow invalidate marriage as an institution. Even the fundamental redefinition of marriage as being about the relationship between the people entering into it (as opposed to their familial/social obligations)-- the emotional bond they share-- became so seamless a part of what we mean by "marriage" that few today would believe it had ever been any different.
3. The existence of same-sex marriage has no effect on anyone else's rights: opposite-sex couples are just as free to marry as they ever were.
4. This is an invalid comparison, as transgenderism and sexual orientation (such as homosexuality and bisexuality) not only have nothing to do with each other, but are in fact antithetical. So concerns about indoctrinating children with genderist ideology should not be applied to matters based on same-sex attraction.
5. Same-sex marriage does not force speech or a credence, any more than the opposite-sex marriages of which one disapproves (for religious or other reasons) do. Just because a marriage takes place doesn't mean that you have to like it, or believe in it. You're free to do neither. Just the same as always.