Argument: Vows are about love, not reproduction; gays qualify
The Standard Civil Ceremony is as follows:
"[Name], I take you to be my lawfully wedded [husband/wife]. Before these witnesses I vow to love you and care for you as long as we both shall live. I take you, with all of your faults and strengths, as I offer myself to you with my faults and strengths. I will help you when you need help, and will turn to you when I need help. I choose you as the person with whom I will spend my life."
The emphasis is squarely on commitment and love, and has nothing to do with reproduction and starting a family.
On this core, clearly gays qualify for marriage, because they can love and commit do commit to each other.