Another argument used to justify the legalization of same-sex marriage is based on the separation of church and state.  A simple statement of this argument would be:

P1:  It is a religious belief that marriage should only be between a man and a woman
P2:  Making laws based on religious belief is a violation of the separation of church and state
C1:  Traditional marriage laws should be changed to include same-sex marriage

The response to this argument is that there are many non-religious reasons for maintaining traditional marriage laws.  Further, P2 is a misunderstanding of what the idea of separation of church and state is really about.