Yet another justification used for the legalization of same-sex marriage has to do with extending the benefits of marriage to same-sex couples.  These might include things like taxes, hospital visitation, inheritance, etc.  The argument would go something like this:

P1:  Traditional marriage couples enjoy certain benefits by virtue of being married
P2:  Same-sex partners should have these same benefits
C1:  Same-Sex marriage should be legalized

The response to this argument is that marriage is not necessarily the vehicle to such benefits.  These benefits could be granted through civil unions.  Thus the same benefits would be obtained without revising marriage norms or laws.