can't say for sure, but this seems rational, and really the only fair-minded thing. to my mind, the husband "sharing", such as it is, consists of his permitting and wholly approving of the sexual liaison between his wife and another man. unless he's "sharing" in the action, that's pretty much it.
ideally it would be preferred if it can be established beforehand that both partners are clean and the potential for pregnancy is not an issue. if not, condoms ought to be required. play smart, play safe. but once they begin, the sharing should be between them, even if the husband is watching. the husband should support that the experience ought to be shaped by them. let their mutual attraction, excitement, and desire dictate how far and how high they go, what they do, or don't do. no outside influence or interference by him. no fear of judgement, no shame for them.
if my wife would want to set prior limits I'd want them respected, of course, and NO always means no at any point. but I'd fully expect a lover to use all his charms to try to tempt and persuade and seduce her to go beyond those limits. to get her to let go and do more with him than she imagined she would. honestly, I'd expect him to want to take everything he possibly can from her, and to want to give everything he can to her, as well. For however long they are together they'd belong to each other. I'd really want it to be their thing.
for instance I can envision a scenario in which my wife, who totally loves anal sex and gets off from it like probably nothing else, might well be tempted to try it, especially with a very well-endowed partner. nonetheless I can imagine her hesitant to share something so intimate and intense for her with another man in a first time meeting. She's only done it with me, and I can imagine her reluctance at sharing such a potentially explosive act -- in all the right ways -- with him in front of me. and then of course there's also the great unknown and the connected issue of trust -- in spite of the excitement, the apprehension she might well feel about taking a super-sized cock there, one wielded by a relative stranger.
But she's an impulsive creature too, sometimes a real thrill seeker. when properly aroused, and in the heat of the moment if the chemistry were there and he was really pushing her buttons and fucked her pussy well, I can just as easily imagine her swooning with lust and desire, so much so that she simply drops all inhibition and totally goes for it, in part to please him, but even more out of her own hungry, horny curiosity, to discover and experience and know ALL she possibly can of and with him in their shared moment; what it's like to be opened slowly and inexorably, stretched deliriously, then filled gloriously, ripe with the possibility of having her body sprung and her mind blown -- in a completely unprecedented way. I would SO endorse and celebrate that exercise of choice, that decision, and that sort of exploration for both of them! even if she'd gone into the encounter with a no-anal stance. Maybe even more so if she had!
great sex is such a fluid, unpredictable situation, the ultimate give and take, as intense and intimate and personal an interaction as two can share. I'd certainly want her to be able to change her mind in the moment, change the rules to suit her shifting moods and desires, and those of her partner. I'd want her to fly. I'd suppose I'd want that for both of them. What's the point, otherwise?
