Maybe not you "must" be for incest and poly marriage, but you should be.<quoted text>
He knows he cannot win that argument, so he desperately tries to divert to unrelated topics and make childish statements like, if you are for same sex marriage that means you must be for legal incest marriage etc.
It is a desperate attempt that he has been failing at for quite some time now.
Marriage equality is marriage equality whether you hate the people involved or not, they deserve the same respect and consideration that you demand.
If people are asking for the same thing you are asking for why is it "desperate" for them but not for you?