Probably for the same reason anyone cares about something not directly related to them: Ideals, religion, ideologies, etc. According to my values, it would be a better world if same-sexed people could marry everywhere, and show their love and be showed respect, not be discriminated against in cases of everything from monetary (tax stuff) practical (like having a ceremony in a church?) stuff. Then again, that is because I see no possible harm or bad effects of this, and if others do, I suppose that it's natural for them to care and say "Nay!".