Please help me give back up as to why incest is wrong and gay marriage isn't?

i know its a stupid argument, but i know plenty of people who think marring your cousin is alright and fine especially if you allow gay marriage! i've tried to say,"DUDE ITS YOUR COUSIN GROSSS!!!" or "DUDE ITS YOUR RELATIVE, GROSSS" but alas i am tuned to deaf ears. i'm not joking there really are people out there who are like this. it hurts my head a little bit. what would be some good comebacks or proof that marring your cousin/relative or animals or anything nasty is not the same as same sex marriage? thinking maybe because people are people regardless of gender, that love knows no gender or color but you probably do have something wrong with you if you are in love with an animal or relative. your chances for messed up baby grows when you don't split up the gene pool! what do you guys think?