You wrote the comment I was about to write, but I would also add: plenty of groups have come up with modifiers or completely new names to describe their marriages as they differ from the civil meaning of "marriage", so I'm not sure what the big deal is about putting the burden on (for example) religions to come up with something other than "marriage" by itself to use if sharing it with lgbt people getting married is just unimaginable for some reason.

See: handfasting, "celestial marriage"/"eternal marriage"/sealing etc. from Mormons (but having different names for their own marriage rites wasn't enough to stop them from supporting Prop 8 I guess...), common law marriage if you want to take the government out of the equation...