There only seems to me to be two logical default positions on this:
- For it - because you are gay, someone you love is gay, you believe in equality or you just really like weddings
- Not bothered
If you aren't gay yourself, why do you care so much what other people are doing, if it doesn't affect you. I have heard some vague nonsense about how it will demean 'traditional' marriage. If your marriage feels less important because of something complete strangers are doing, then the problem in in your home.
Then there is the religion argument. Apparently, god won't like it. Again, if it isn't you doing it, why should that worry you so much? Or is your god the type to smote everyone in his anger? In which case, I'd be questioning whether that is the kind of god I want to hang out with, rather than worrying about what Adam and Steve (or Adele and Eve) are doing.