I’m pretty sure they’re saying that a wedding would be a sham if one party was already married to someone else, which doesn’t seem to be the case here?

Regardless, to your point about different religions/countries, there are a lot of people in the world who have had to fight long and hard to gain the legal right to get married, and a lot of people who STILL don’t have that right. There are so many benefits that go along with legal marriage, and there can be a lot of emotion (or pain, in some cases) associated with that legal recognition, even for people who have the privilege to take it for granted.

People naturally relate things to their own life, because we’re all human and we’re all a little self-centered. That’s the best explanation I can give you. I don’t know if you’re going to get reasons that you consider “good,” or “logical,” but I also think emotions can be valid even when they’re not necessarily logical.