there is not really such a thing as married-people-only spaces

I suspect many single people would dispute that! But I do see what you're saying, and for all practical purposes I'm in agreement. Such disagreement as there is lies precisely at the impractical, or as you put it conceptual, level. Because same-sex marriage changes the concept of marriage (at least for those whose concept of marriage did not previously encompass it), it changes the self-conception of those who "identify as" (unlovely term) married. Changing one's self-conception is hard work, especially for those who aren't used to that kind of labour, and to that extent it may be worth attempting to help the process along by showing that the new conception is a more generous, more interesting and richer one. But it's still a side issue, I wouldn't dispute.