I thought my comment "The thread's about gay marriage, not about gay "relationships" per se, far less gay sex in general. Even the thickest of you should be able to work out that a person can "disapprove" of the first without necessarily disapproving of the the other two?" should have made half of Kromo's post and all of Mikey's unnecessary.
If we want, however, to make this an issue of rights ("marriage equality" as one poster put it on a different thread - same topic) then there's a rather more intelligent debate to be had.
You might start out by arguing on identical premises logical and emotional ("denying love between two people" etc.) that incestuous marriage should be legalised. I'd love to hear a logical argument agaisnt that.