Everything you and stormdog say about what is claimed as "an effect on hetero marriage" is a change in the concept of marriage. This is not denied by supporters of same-sex marriage. It is not, however, an effect on hetero marriage or hetero marriages. Opponents of same-sex marriage have repeatedly claimed that it is, and they have repeatedly been unable to substantiate this when pressed, and nothing that stormdog says changes that.

The real belief seems to be that marriage is a sort of club, and they want to keep the riff-raff out. But married people as a group do not form a club. Each marriage is an individual club with two people in it (if it's not poly). Even children are in a sense external to the club of the married partners (which is one reason why the post-facto claim that marriage is all about procreation is nonsense).

This is where there is a slight difference between [trans women/women] and [same-sex married people/married people]. Married people don't form a club, but women do. There is such a thing as women-only spaces; there is not really such a thing as married-people-only spaces. The argument that trans women don't belong in women-only spaces seems to me to be based on fundamental misunderstanding of trans experiences; but I can see the relevance of the concern, in a way that seems utterly irrelevant to hetero married people looking at homo married people.