As been said above, adultery is justified as a means of escape if the party in question does not have an option of divorce. If such option is present, however, then why not use it and then
be with one's crush with clear conscience?
Also, in the times when quite a lot of marriages were forced and at least one side did not really had an option to refuse, adultery was absolutely justified. It's not like adulterer ever agreed
to stay faithful to their partner in the first place!
Another situation when adultery is justified is when the partner is also cheating. One who cannot hold their own promises does not deserve promises made to them held.
By the way, it is specifically betrayal of trust and breach of promise (which is a big deal
to this one) that bother this one about adultery. Not the act of sleeping around in itself. So personally, this one would not be likely to forgive her partner for adultery unless they confessed without prompting from this one. Nor would she commit adultery herself. And nor would she accept jealousy and attempts at control, because if partner does not trust
this one, what in the Nine Hells are we doing together?! Nor would this one be
jealous and controlling, because if this one cannot trust her partner in such an easy matter, why does she trust them to be in her home and not murder her in her sleep? But at the same time this one would be quite comfortable in an open relationship, or any other arrangement that allows multiple sexual partners openly and with prior consent of the spouse