Tomodachi Collection users have found that they can allow two male characters to tie the knot, but the feature doesn't work with two female characters. As such, it seems Nintendo is now patching out same sex marriage in full.
Here's a grab:
Note the line about human relationships. Nintendo Life also suggests that the same-sex marriages for males could be unintentional, and that Nintendo is now keen to remove the feature as a result. Either way, the feature has seen new gamers flock to the game, so it will be unfortunate if they're then unable to do as they please.
We'll have more on this one as it plays out.