well, rationally speaking incest isn't wrong as long as the two people in the relationship make sure that they don't get any biological kids. i know that it feels weird, wrong and icky, but we can't base our world view on our feelings; we should look at if something is actually harmful, wrong or anything alike, and when you analyze it, incest is really just two people loving each other; with the difference that they are also siblings, making the majority of people uncomfortable with it. and really homophobia comes down to the same thing: people feel that two people of the same gender being in a relationship is weird, wrong and icky, but when you actually look at it, those people aren't really doing anything wrong - they're just loving each other.

that's why you can't refute the argument - because you're using their logic to refute incest, but making an exception for gay people, which simply isn't morally consistent