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.