Frankly, I do not find the "because other countries/areas/states are doing it" argument at all telling, on either side of the issue.
What I do consider important is that I cannot personally think of any reason to prevent it.
"because we didn't before" is just dumb.
"because god says no" is unconstitutional.
"because I don't like it" is bigoted.
"because marriage isn't same sex" is also dumb...You can't use one current definition of a word/idea to prove the argument for changing that definition wrong. That's just insane.
In short, while some lingering dislike for the idea exists in my mind, there is no legit reason to prevent it.