Ok let's just focus on the sciencier one for a second. Say someone doesn't like incest because it's bad for the gene pool. Their icky emotions about it predate religion.
That isn't a slippery slope to eugenics. Inbreeding depression is real, but eugenics is discredited as unscientific. We already know that rules against incest don't lead to rules against people with defects reproducing.
The problem with this line of thinking is that you're expecting people who support gay marriage to convince you about some other thing. 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.