Instead, agree with it. And then just take it to the next level and make them debumk their own argument.
For example, Yes they are the same. After all at the heart of it gay people don't choose to be born that way; therefore, we shouldn't discriminate against them. Get them to agree to that (and they will). But I think we need to take it a step further to get rid of all this hatred spurred by religion. I'm certain pedophiles don't choose to like little boys and girls, they are obviously born with that lust in them. So who are we to judge him? We need to live and let live, especially if the kid says ok. By then they'll be shocked and want to dismiss you.
I'm sure they'll say the difference is that they are both consenting adults, but you can't let them off that easy. Because the core of their argument is that they are born that way and we need to embrace them for who they are. Just keep framing the discussion around that and they'll get mad and never talk to you again. That's how I keep stupid liberals (I know that's redundant) at bay.