I don't think group experimentation will kill a relationship, but I think enough one-on-one encounters without the other person will, especially when one person has paranoia and trust issues to begin with.
And I think I'm right and that you're wrong. It goes in circles because neither of us will be able to convince the other that they're wrong or we'll be too stubborn to openly admit we had a change of opinion.
I've known couple who have been together for several years and who love each other and sometimes have group sessions with friends of theirs, but their love hasn't waned.
I don't think its healthy, and it would ruin any relationship i was in. I coudln't stand seeing my lover look at someone else in a sexual way...eh not happening. ugh, actually almost upsets me to think about it . although i guess that if i thought it would make him happy i'd do it, but then never want to do him again. lol
I'd have a major problem with him wanting to do it without me, honestly. At least if I'm there, I can say I monitored in some way.
What does it matter? If he's messing around on someone without you there or messing around with someone with you there, he's still messing around with someone either way.