Honestly I don't understand why same-sex marriage is such a big deal. Blowing it up like it's some big whoop isn't going to fix anything. Just call it 'Marriage'. Geezus.
Honestly I'm cool with it real world. Because there are a lot of systems in place for both same sex and opposite sex marriages to prevent both falsified unions and unwanted unions. By allowing same sex marriage in a video game you will just ruin it for those that are in a real life same sex relationship. You will end up with a lot of people doing it for the laughs. So I would suggest putting in shared retainers, forced membership in free company and grand company. Shared wallet etc etc. I would also make it so the only unique items are the rings which I hope can be glamoured, and the actual wedding event. Dresses, tux, etc. should all be accessible otherwise you'll have people creating characters just to get those items which defeats the purpose of the whole marriage idea.