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.