I suspect that there are good game-theoretical/TDT reasons for the rule that one shouldn’t break promises, so if Alice has promised to Bob that she won’t have sex to anybody else, I’d say it’d be wrong for Alice to have sex with Charlie even if both Alice and Charlie are consenting. (But the idea that people should never have sex unless they promise each other to not have sex with anyone else I do find silly.)
I suspect that there are good game-theoretical/TDT reasons for the rule that one shouldn’t break promises, so if Alice has promised to Bob that she won’t have sex to anybody else, I’d say it’d be wrong for Alice to have sex with Charlie even if both Alice and Charlie are consenting. (But the idea that people should never have sex unless they promise each other to not have sex with anyone else I do find silly.)