As I've said before, it really depends on the in-game benefits. In America marriage has legal benefits like a lower tax rate. If there are no benefits besides a ceremony, many players will end up holding their own.
That being said, I'd prefer that same-sex marriage be allowed. It's not like you really noticed marriage all that often in XI. It took place in areas that wouldn't conflict with others' gameplay (such as the mansion at the far end of Southern San d'Oria or the top of the lighthouse in Gustaberg). I actually missed the one wedding I knew about, and never saw any others before the feature was discontinued.