Chief Justice Roberts takes issue with Ted Olson (pp. 36-37 of transcript):
I’m not sure that it’s right to view this as excluding a particular group. When the institution of marriage developed historically, people didn’t get around and say let’s have this institution, but let’s keep out homosexuals. The institution developed to serve purposes that, by their nature, didn’t include homosexual couples.
It is — yes, you can say that it serves some of the other interests where it makes sense to include them, but not all the interests. And it seems to me, your friend argues on the other side, if you have an institution that pursues additional interests, you don’t have to include everybody just because some other aspects of it can be applied to them.