memes

Their icky emotions about it predate religion.

You're going to have to prove that one because religion has existed alongside humanity for all of known history and marriage of first cousins is still common in the Arab world.

We already know that rules against incest don't lead to rules against people with defects reproducing.

Says who? We've only just started our journey down the path of total secularity. We have no idea how this is going to play out. China's CRISPR program has already demonstrated that gene editing is possible and Canada is letting people kill themselves. Brave New World already imagined how these ideas will run wild once free of the baggage of the past. Secularity has no moral construct. All options are on the table including the sterilization of people with hereditary defects.

You're expecting people who support gay marriage to convince you about some other thing.

No I'm saying that gay marriage crosses the rubicon. It is a complete departure from what marriage means in any historical or religious context. Which begs the question of what it is and why it even exists. If gays can marry despite prohibition across all cultures for all of human history then where are the limits? Who sets those limits and why to they get to be in charge of who gets to be married? It all falls apart you see.

And if any of these social taboos are actually a good idea, then you're lumping gay marriage in with them, like comparing gays to pedophiles.

Don't dismiss the argument because you are uncomfortable with the possible implications. Contend with the premise. You drew that conclusion not me.