Interesting sort of question haha I gotta say that I love MMO's and as for free ones, I'd go RIFT or ESO (not that they promote the same sex relationships etc - but that they allow it). On that note - ESO allows characters a "scroll of Mara" I think? This allows you to marry another character. I haven't checked the same sex allowance though. Also - These games are good MMO's too so that's a bonus.
Have you tried Archeage? It's free and Korean (and is available in the "Glyph Client" for Rift) and probably has this. It might not tho. It's fairly P2W which detracts from the enjoyment overall.
Gooooood Luck!