I'm serious You guys need to be progressive.
In real life, I'm so 'progressive' that I want an end to all government involvement in marriages and legal rights for parents other than a birth mother to be at the mother's sole discretion (that's deliberately flexible language, no DNA relationships required, no limit on the potential number of legal parents).
But this is a medieval-toned game. It's actually more honest about the history of marriage than any of the culture war debates with their dependence on extremely modern notions of romance and childhood. Traditional marriage among the poor folks was about establishing productive households. The nobles add wealth and status to the exchange, but still marriage was a 'business' thing for the most part until the late 18th or 19th century in some parts of the world (and not everywhere, yet).
Basically, even though Raven has a point when it comes to past real-world monarchs (almost all male) having numerous sex partners of varying social ranks, including that in the game would make for a very messy UI, even if you limited it to something like 3 legal spouses per royal person and a 1-per-realm House of Concubines improvement.
Also on a pragmatic level, while I suppose same-sex couples could provide some basic diplomatic value, including that in the un-modded UI would probably cost too much dev time for the number of players who would use it (I'm assuming maximizers are the majority, so forgoing 'free' champion units would make no sense). And I'm not bashing at all here; I'm a gay man.