Not to quibble, Undertoad, but...yeah, okay, maybe I am quibbling, but...I think a classic 'straw man' argument would be if one of those folk said, "The people who are in favor of gay marriage say it's okay for anybody to marry anybody else, even relatives, children and animals."
Maybe you've heard somebody say that - in which case you're absolutely right, that IS a 'straw man' argument. I'm just saying that the way I normally hear it phrased, it's definitely more into the 'slippery slope' variety of logical fallacy.
Hey, check this out.
http://www.datanation.com/fallacies/index.htm