Here's that question

If you're only FWB, then by definition he can't "philander." You cannot cheat on a non-existent committment. He clearly doesn't want further hookups or -- Gawd forbid -- a relationship with you, and so he's telling you the only way he knows how, demonstrably and crassly. 'Twould be nice if he could tell you the adult way--you know, with his words, but that's apparently not the case. ..
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If I were you, I'd take the high road and approach him yourself. Say something like, "You know, Apollo, I enjoyed our little assignation the other night, but I've no desire either to repeat it or to invest in a white picket fence with you. You can knock off the tongue Olympics with my buddies -- or at least be respectful to those of us who have sensitive stomachs and take it private." ..
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All right, that might be a little snarky to qualify as the "high road," but you get the idea...

It is understandable that you woudl feel uncomfortable and even a little annoyed, but if it was drunken fling then he really has no commitment to you and can do whatever with whoever. I think you shoudl casually tell him that it makes you and the rest of your friends uncomfortable with his PDA and ask him to keep it down a notch for the sake of your friendship...

Sounds like he is uncomfortable with what happened between you two. Maybe he's trying to play it like he's flirty with everyone? Not the right move, but there's no accounting for what someone will do when you sleep with them casually and have no clear arrangement...

I think you should talk to him about it. You can tell him that you don't mind if he sees other people -- you know that you're not in an exclusive relationship. But you'd really prefer it if the other people not be your close friends. That is a reasonable request to make -- I've made it before. However, I heard of a story recently where a girl told a guy that -- but then the guy really liked the girls best friend and he wouldn't agree to it and he ended up going out with her too. And girl number one flipped. So, here's the choice: You can tell him you'd rather him not sleep with your friends. But, you need to realize he may not agree. And in that case, you need to walk...

I think the question is: Did he ever flirt with and get really physical with your friend BEFORE you two had sex? ..
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If he didn't, then he's taking a pretty immature route to express that he's either nervous about or uncomfortable with what happened and you have every right to take him aside and tell him to cut it out. Or at the very least, ask why he's acting this way. ..
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If he did, then well, there's nothing you can really say, because it's a bit of a "well, you might have known better" sort of thing. At the same time, since you are all friends, you should ask him not to make you feel so blatantly third wheel. It's not something you should be doing to friends anyways! ..
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Also, have you talked to this mutual friend as well? She might be able to help, or at least, you can ask that if she has to make out with him, please not to do it in front of you...