Also, I think it is confusing to say "cultural customs and rituals are not rights and don't have to be inclusive." That is largely correct from a legal perspective, but still makes for a bad society to live in. You probably legally can found a club for white people only, but it's unlikely that that would be a nice club to be in. And also, just because something has a long tradition, that does not make it non-discriminatory.
Whether some gay people don't want to marry obviously is completely irrelevant to the discussion, as this is not about forcing gay people to marry. This is about the freedom for those who want to, not about forcing those who don't want to. And even if some gay people feel that it's not appropriate for other gay people to marry, it's not the job of the state to enforce that on those other gay people who actually do want to marry.
And I do agree that you don't have to carry any ill-feelings towards blacks in order to want to preserve the traditional definition of who has to sit where on the bus. But that completely ignores that there are people with feelings on the other side as well - people who are excluded, people for whom sitting on "black only" seats is not just a quaint tradition. Those strong feelings about traditional roles do exist and totally are real, be it about the role of slaves, of black people, of women, ..., but that does not mean that living them does not hurt other people. We as a society can (and have to) force people to confront that they are actually causing harm with their clinging to traditional ideas, often for totally irrational reasons. That does not mean that we cannot empathize with those people who sincerely believe that they are losing something when giving up such strong-held beliefs and ideas, but we also can not accept that they hurt other people.
And, really, I would want to encourage you to re-read what you write, replacing everything in it that is about gays with black people and mixed marriages and other things that we have figured out a while back, try to read it as a sincere argument. You are used to the idea that marriage is a custom specific to heterosexuals. But you have to consider that people at some point were equally used to racial segregation. There were people using the exact same arguments back then, feeling equally right about how traditionally marriage was for "people of the same race". And yet I would hope that the suggestion of a "interracial union" for those mixed couples who want to get more or less the same benefits that "one-race couples" enjoyed traditionally (so that the traditionalists can keep their tradition of what they perceive to be a real marriage) would seems utterly absurd and inappropriate to you. Then try to extrapolate from that perspective on the past to how you would view this current discussion as a person in the equally (not that) distant future. And then reconsider who is undeservedly labelled a bigot or ostracized.