There is no way to make the case that gays shouldn't have equivalent marriage (or other rights) without an underlying belief that gays are lesser human beings. You really don't have to be that clever to see that gay marriage doesn't correspond to group marriage, marrying underage people, etc. The real analogue to 4 men marrying one woman or 4 women marrying one man is not a gay couple getting married, but rather 5 women (or 5 men) marrying each other. Failing to see this obvious parallel and lumping gay marriage in with group marriage, marrying cousins, etc., can only reflect a presupposition on the part of the questioner that gay = deviant. To stick to the question once this obvious flaw in it has been pointed out would reflect the fact that the questioner isn't really looking for an answer, but is just trying to stir up shit with a kind of low-grade sophistry.
In terms of only supporting marriage because it produces children, besides the fact that childless heterosexual couples do enjoy equivalent rights and no one is proposing to take them away, that is ridiculous also because there is no need to promote births in the US nor in most countries. If anything, incentives should be given to people who don't have children. And even if there were some kind of burning need to create more babies, stopping gays from marrying would do zero to ameliorate that.
If mixed-race marriages were illegal, racists (both overt and covert) would raise the very same objections: "Where do we draw the line??? The mixed-race marriage activists keep saying it's different than a man marrying a donkey, but they never offer any proof!"