Yeah, but with a "normal" relationship people don't do such deep introspection. Most people just go on their feelings. I think the OP should have the same liberty. Maybe people should do more evaluation before going into a relationship, but if the OP wouldn't do it normally, nothing should change the game because it's his cousin.
9% chance of mental handicap with cousin couples, as opposed to 3% for normal couples. Not a huge increase. And since you're already pretty unintelligent yourself, you have nothing to brag about.