They have no control over their own lore/worldbuilding. So might as well marry 10 people. Think the only limitation is gameplay and they want you to pay for each one

Gay couples can marry, so might as well assume that its also lore accurate. The lore is full of contradictions at times though.

ARR had a gay couple, that was discriminated against in the story, but I have no idea if it was because they were gay or from different cultures / races.
The races mixing narrative is bullcrap though cause we see multiple races mingling. A miqo hyur couple in ala mhigo talking of starting a family. At least a couple of hyur hitting on lalafel / gegerujus Costa Del Sol harem. Who knows how many prostitutes from different races.
That Highlander dude with his two Miqote in Thanalan.
I think only Ishgard is against mixing (Hilda)