One argument for legalizing same-sex marriage is the claim that traditional marriage laws have no rational basis, and are instead motivated only by hate and prejudice.  The argument could be stated this way:

P1:  Traditional marriage laws have no rational basis and are motivated by hatred
P2:  Laws must have a rational basis
C1:  Marriage laws should be changed to include same-sex marriage

P1 ignores several rational arguments for not legalizing same-sex marriage.  There are several listed on the Discussing Marriage web site.  The primary one in my view is the promoting of permanence and fidelity in a traditional marriage with the potential of procreating.