It's ignorant, places way too much emphasis on sex rather than committment and love, and is justified by stupid ideas like marriage being "reserved" for heterosexual couples for the purpose of procreation, but it's rational. It also leads to seperate but equal legal frameworks, which are bullshit.
Bans on interracial marriage were irrational, not relying on a single objective fact.
What I would like: abolish state-recognized marriages altogether. Civil partnerships for everyone, let churches squabble about marriage if someone wants a ceremony, and they can enter into a partnership for benefits.
What's practical: modifying our legal framework for marriage across the states.
> they can be rational and maintain that a relationship between two people categorically incapable of producing children together biologically, two people of the same sex, can’t be a marriage
Pretty much nobody actually makes this argument, though. Probably because infertile people get married all the time.
Not really, as I said before there’s no biological basis for refusing to accept an interracial relationship as a marriage.
I'm not going to continue defending views that aren't my own, so if you're interested in reading what exactly "traditional marriage" defenders are talking about then try this review: http://www.harvard-jlpp.com/wp-content/uploads/2013/10/Georg... (pdf)
That's explicitly not rational, unless you are going to deny marriage rights to opposite sex couples who are "categorically incapable of producing children together biologically."
Incidentally, the idea that marriage has long social and cultural roots is also kind of dubious. If you look at marriage in 1800, it's essentially an entirely different arrangement to marriage today; notably, it lead to effective legal erasure of the wife as a person.