I think the real importance here is that it shouldn't be brought up
just before the wife and lover are heading to the bedroom, or whatever. This is something that either the couple, or more preferrably, the wife brings up in the
initial talks with the prospective lover. There's going to be a certain level of apprehensiveness anyways on that initial meeting, and you don't want this new guy to have any doubts as to the intent of the husband being there, or you risk it affecting HIS ability to perform. So, get it out front
early to see if the lover has issues with hubby being there.
Although I've screwed a few married women in my time, I've never been in the situation where the woman's husband was there
watching me ... in the couple swaps my wife and I have done, the hubbies have always had their hands full entertaining my wife. So, if it was me, as the 3rd wheel lover, and it was the husband's intent to sexually interact with ME during the MMF, it probably would affect me, and I'd probably pass on the opportunity.
Yeah, call me a wuss or insecure, if you wish, but that's just me. Mac