No, you're wrong.

As for badmouthing someone you're (?)sleeping(?) with: I dunno, maybe we just have different perspectives of life, but that's not unusual behaviour in my mind.

Man, to me it's total asshole behaviour.

they know about each other

They didn't though? She didn't know he was still talking to the ex, he said he wasn't. He clearly, given the text we're actively discussing, was telling the ex he wasn't in contact with OP. So sure they knew each other existed, but he specifically told both of them the other didn't matter to him. You don't see how that's complete asshole behaviour? Cause it really really is.

it's not outside the realms of possibility that you say some shit to one of them about the other.

Again, asshole behaviour.

edit: do you also think someone is a scumbag if they lie to someone to sleep with them a la "I'm a fighter pilot"

Well, yeah. That's a total scumbag asshole move, are you trying to argue it's not? lol. But there's also a difference between lying to get laid one time, and badmouthing someone you're pursuing a relationship with.

It's honestly really fucking disgusting to sleep with and actively pursue someone than badmouth her to someone else. If you're not exclusive, I mean fuck around flirt around whatever. But it is total asshole behaviour to say shit about