When one of my partners was having trouble with his girlfriend at the time over this poly thing, I asked about why she doesn't date other people and with the intention of discovering if he was demanding conditions that were not conducive to her having other successful relationships. I knew she was straight so I knew it wasn't a One Penis Policy, but there are other things he could have been doing that halted her chances. This would have bothered me and it wouldn't have been something I could live with.
How this relates to your thread, OP, is that if I were you and I felt that my boyfriend had a OPP that actively stopped my husband from dating his wife, I would probably misrepresent the way I felt about that. It would probably seem as though my objection was because it was super important to me that my husband dates his wife so we have a quad, when that couldn't be further from the truth. My issue would be the fact that he has such a rule and the red flag that presents for rules he may instate in our relationship. If he needed those rules with her, but not with me, that would actually make me question why he doesn't have the same kind of fear around me. I mean, it's not a positive thing at all but if he is scared of some guy replacing him in his wife's life, why doesn't he feel the same about me being enticed away?
Tl;dr: I couldn't accept my partner having a One Penis Policy in his other relationships. Not because it stops my other partner's dating his partners, but because those sort of rules are red flags.