Another argument used to justify the effort to legalize same-sex marriage is the argument from humanism.  It is stated that without state licensed marriage, same-sex couples are doomed to a life of loneliness.  The argument would be something like this:

P1:  Marriage is necessary for loving, intimate, fulfilling relationships
P2:  Same-Sex couples also want loving, intimate, fulfilling relationships
C1:  Same-Sex marriage should be legalized

The response is that loving, intimate, fulfilling relationships do not require a government license.  Same-sex couples could have just such a relationship, whether the government licenses their relationship and calls it a marriage or not.