I'd describe that as monogamous. You're saying that you think you'd be able to negotiate a new rule if circumstances arose, but the current rule is monogamy.
Mm. OK, with that connotation of "disallowing", I would agree. It's not the connotation I would expect to ordinarily come to mind in conversation, and in particular your statements about "B wishes A would not sleep with others" emphasized a different understanding of "disallowing" in my mind.
Another month has passed and here is a new rationality quotes thread. The usual rules are: