Aside: I use the phrase "traditional marriage proponent" because that seems to be what people for heterosexual-only marriage call themselves. I think some parts of their stance may not be traditional depending on which period in history we look to for examples or marriage, but for simplicity's sake I'd rather just adopt people's self-descriptions and get to the actual arguments.
People seem to think that marriage must mean either (1) heterosexual couples making lifelong commitments and producing children or (2) any consenting adults making lifelong commitments to love each other. In terms of government recognition and enforcement, I think both of these definitions are problematic.
When I talk to a TMP, the conversation often goes something like this (roughly paraphrased from multiple conversations):
TMP: Marriage requires a lifelong sexual union that bears children.No no no. Setting aside the gay marriage debate for a moment, this is not how it should work. People should be able to defend their perspectives regardless of what my perspective is. I could think marriage should be between dogs and cats and have completely idiotic reasons for thinking so, but that doesn't mean the holes in the TMP's arguments have disappeared. Shouldn't it bother people if there are holes in their arguments they can't reconcile?
Me: Then why do we have [no-fault divorce/legal infidelity/infertile couples marrying]?
TMP: Well then what do you think marriage is?
I suspect TMPs think the options are either the TMP definition or the gay rights definition, so as long as they can poke enough holes in the gay rights definition, the TMP definition has to win by default. But I don't think that's the case. If I find holes in both definitions, why would I go TMP by default? If I hesitate over some aspect of the gay rights perspective on marriage, how would that prevent me from continuing to see holes in the TMP view as well?
Whether we're talking about gay marriage or any other topic of debate, I guess all I'm really trying to say is: defend your position or stop spouting it.