Thanks for all the replies and support, girls
Just for an update.. after a long fight/discussion with him, I've finally got him to understand why I had a problem with this setup. He had been clinging pretty strong to the "well, I'm not going to do anything, and I told her about you, so it's fine" thing, so I had to kinda break it down for him. After working through a pretty web of lies designed to "make me calm down", I came to the conclusion that he's obviously not the respectful type.
I do trust him, but I hate feeling disrespected and I can't see "being a nice guy" as a big enough reason to remain friends with a recent ex (who is clearly not a good friend). This one's cut loose, in any case.. it's amazing how corrupt your viewpoints can become after listening to someone try to rationalize their behaviour repeatedly. Again, thanks for the words.. sure glad I'm not the only "crazy gf" in this world, haha.