"It's about love."
If this is the main argument and we allow gay marriage, then anyone proclaiming love (beastiality, pedophilia, for example) should be allowed a marriage license.
"It's about freedom of religion."
If this is the main argument and we allow gay marriage, then anyone wanting to refresh the definition of marriage due to religious beliefs (polygamy, for example) should be allowed a marriage license.
The focus should be on showing that gay marriage is maturely consensual, monogamous throughout life, a healthy alternative to any other lifestyle, and has the potential to successfully raise children if desired.
In my opinion, such a standard is what marriage is all about: a mature, consensual, monogamous, committed through thick and thin, healthy, responsible, parental relationship.
I suppose one could argue that "able to produce children within itself" could be added as a criterion.
And, since we would have to go off of generalities and the healthiest & most exemplary versions of relationships suing for the title of marriage, barren heterosexuals would have claim to marriage.
The question would be, though, whether or not the ability to produce children is essential to a successful marriage.
Does the ability to produce children make a couple more monogamous?
Does the ability to produce children make a couple more committed?
Does the ability to produce children make a couple stronger parents?
Does the ability to produce children make a couple more responsible?
Furthermore, it could be questioned whether or not the ability to reproduce within the couple itself is essential -- as opposed to using other means of reproduction, such as surrogacy.
Anyway and in sum, gay marriage advocates need to more clearly argue that a gay couple holds all of the essential attributes that contribute to a successful marriage (which would require a solid argument as to what contributes to a successful marriage).
Sure, love is important. It's also vital that standards do not respect any religious beliefs. However, those are just small pieces of the entire argument.
Granted, there have been some significant strides in the legalization of gay marriage, and I'm thinking it's going to be country-wide sooner than I had originally thought. But I think we need to do a better job arguing the case in order to ease the anxieties of those so ardently against gay marriage.
Granted the second, some people simply cannot be convinced -- those immune to logic and reason.