I'm going to do a random thing and post a response without reading other responses.
To me the issue would be my partner lying to me, that would be the only issue for me actually. 1. why did he lie. 2. if he had been honest ahead of time, what would my response have been to his desire to have casual sex at sex parties, if he had told me after the fact, how would I deal with the issue in the future?
All the other stuff comes after the lying thing, so I can't see even dealing with it until the big issue of lies was settled.
So since I don't know why he lied since I didn't check any follow ups.. I'm going to just babble some thoughts (took a big dose of Nyquil awhile ago to deal with my head cold). My ex cheated on me when he could've honestly slept with any of thousands of women because we'd opened our relationship, however while he had liberal boundaries (use condoms, don't sleep with people who are cheating, don't sleep with 3 women who tried so sleep with you when they knew you were in a monogamous relationship) he broke all of those, and then lied about it for 9 months. The lying was the main thing, if he'd come clean ASAP..if this..if that...nevertheless the thing for me was that he lied, and looking out for me instead of my sexual health/safety had dropped from #1 on his list to #whatever. Therefore being in a relationship with him was no longer tenable. If you think your sexual health isn't up there at #1 for your partner, my suggestion is to think hard about the relationship.
Ramble #2. My current husband has had group sex a dozen or so times. Most of them were with friends, but I still have trouble wrapping my head around it, since it hasn't ever been a part of my life. I know I'm unlikely to have a 3some or 4some with my husband since his libido/attitude isn't "sex is awesome!" so group sex is likely out for anything that he is involved in, however I understand that in the future he might be open to it, and I know I might be open to it, but if it is happening with us at the same time or at different times, it just wont be happening without a discussion.
Nevertheless ...hmm, could I be not jealous of my husband having group sex with other people? If I had said I was fine with it yes, if it was a party
and if I wanted to be involved in being there, but I wasn't invited and he wanted to do it anyway, probably not. For ME, I'd have already negotiated the FUCK out of any group sex situation before I had it, so my husband would've already had 30 chances to object.
If I was going to give my blessing for my partner to live his life as he saw fit and stay out of it (not my default for a marriage btw), if he wanted to be involved in casual hookups, because of STI risks (genital warts, extra high risk HPV strains) I'd still insist on condoms for intercourse, and I think if he was going to have sex with people he didnt know well enough to ask about last STI results with, would probably default to condom use with him regardless.
OK, now off to read through the entire thread and likely wish I hadn't said half of the stuff I did say