Responding to "Homosexuality is wrong, the same way incest is wrong"

[Esquilax]

"Well, you know? How about we allow sex in the family? If it is made in consent, and no one is hurting anyone, why wouldn't we approve it?"
To this I wasn't ready. I never really thought about it.
He sensed that, and a big smile of victory and satisfaction was on his face, while we were entering the coffee house and had to leave that at it.

There is so much wrong with him thinking he's victorious from that argument, mostly because there's like nineteen hidden, unsupported premises in it.

For one, he's making a comparison between incest and homosexuality based solely on the wording of your earlier argument, and not on the characteristics of the subjects he's comparing. Sure, he said the same thing you did but with a different word, but where's the actual connection that links the two of them together? Where's the actual valid comparison that would lead you to think this is an actual slippery slope he's pointing out? If there isn't one, and he certainly didn't even attempt to provide one, then you're entirely justified in treating homosexuality and incest as different things, and that acceptance of one on the basis of your argument does not necessitate the acceptance of the other. It's like if he'd gone off of your argument but replaced the word "homosexuality" with the word "murder." You'd be quite right in asking him what murder has to do with homosexuality, and nowhere in what he's said is there an actual point of connection... so why should you take what he's argued seriously at all?

Mind you, that's just the way in which his position fails as argumentation, there's also the unspoken assumption that incest, in all cases and at all times, is wrong... which is something else he probably doesn't have the wherewithal to justify beyond intuitive ickiness. I'm not saying an argument couldn't be made for that, just that he hasn't made it, and therefore a key portion of his argument is unsupported; why shouldn't we approve of incest, all things being equal, and with consent and a lack of harm implicit? What's actually the argument there?

So, basically, both the content of the argument, and the argument itself, is faulty, or at least unsupported. That's enough.

So if there is an increased likelihood of having handicapped children, it should be outlawed.

Wouldn't a consistent application of that logic also preclude, say, handicapped people from having children, where that handicap is inheritable? Where do you draw the line, in terms of what's an acceptable likelihood of genetic defects?