Again, in casual conversation where no one was really debating, it's obnoxious. When you're talking to friends in real life and they say something, do you request sources from them? No, because it'd be rude and annoying. If you were debating them in earnest and you both disagreed on something, sure, that would be expected.

But that wasn't the case here, the initial statement was common sense: If pedophiles are allowed to meet up and trade AI generated child sex abuse material, would that cause some of them to be more likely to commit crimes against real kids? And I think the answer is pretty obvious. The more you hang around people who agree with you, the more an echo chamber is cultivated. It's like an alcoholic going into a bar without anyone there to support them in staying sober.

Anyway, it's your opinion to think asking for sources from strangers in casual conversation is okay, and it's mine to say it can be annoying in a lot of circumstances. We all have the Internet at our fingertips, look it up in the future if you're unsure of someone's assertion.