Or for sacrificing your first virgin dither to a volcano, depending on your religion. I personally don’t care what beliefs people may have as long as they don’t try and apply their rules to people not of that religion.<quoted text>On some people's list, religion is the number one reason to keep marriage between one man and one woman. On my list, religion is number 26.
Kant offered a way to decide if an act or policy is moral without using religion; ask 'What if everyone did it?' If making a policy or an act a universal law produces bad results then the policy or act is bad. If same sex marriage was universal it would mean an end to mankind or an end to monogamous marriage.
Reason 13 for keeping marriage male/female; Kant's Categorical Imperative.
Your other point is moot, not everyone is gay, I don’t happen to be gay and would have no interest myself in a same sex marriage, but I have yet to hear a single valid reason why others that do happen to be gay should not be able to marry.