Generally, no, I don't think it's ok. Are there exceptions? Probably. On that springs to mind are poly-amorous situations where partners are able to have other liaisons but perhaps don't feel the need to discuss them. I've had (pre-DH) similar open arrangements where it was agreed that each could do whatever they liked provided they were safe- then it was don't ask, dont tell But that kind of thing is probably not what you have in mind. I'd never knowingly be the other woman, no. I don't think there is any legit raitionalisation for it- anyone who does rationalise it is likely kidding themselves.