I live in a world where being involved with a married man is not normal, and in fact, is wrong. I GET intellectually that my BF's wife's knowledge and consent change a great deal. But it doesn't change my emotional response. Just a suggestion for how he's feeling. He may have no thoughts of pretending he's with a single or cheating woman.
To be asked to sit face to face with someone when I've been physically active with her husband would be the emotional equivalent of being pressured into doing sexual things with which I'm uncomfortable. If he were to tell me it's either dinner with her and him, or break up, I'd say, "I've treasured every moment. Bye."
I might also strongly suggest that if this is a hard boundary for them, they stick to dating people within their poly world who feel the same.
I can also see how, for someone not used to this, the demand that, "My husband insists he must meet you," might come across as a bit of a possessive thing, as the husband making sure the newcomer knows his place and that he better not step out of it, knows who's really in charge here. It would certainly feel like the husband calling the shots in this guy's relationship.
I don't get the feeling at all that this is what you intend, but you have to remember, you've invited this man into a world with which he is totally unfamiliar, a world that turns convention on its head and throws everything he knows about marriage out the window. In many ways, he's walking in the dark, not knowing what to expect.