If only having marriages for heterosexual couples is unconstitutional, I don't see how allowing marriages for homosexuals as well can suddenly make marriage constitutional.

OK so homosexuals were denied the right to marry in the past, but what about asexuals* or people who can't get married for various reasons? How are they being discriminated against any less than homosexuals were?

* Yes, people will point out that asexuals can marry, but that is akin to the old argument that heterosexual-only marriage laws didn't discriminate against homosexual men, since they could still marry a woman like any other man!