If someone regarded as a boyfriend or girlfriend is perceived to be unfaithful strong forces are generated to break up the couple. This is also a consequence of The All or Nothing Syndrome of Girls' Behaviour in Courtship. Here we saw that a complicated pattern of increasing degree of intimacy must occur for the couple to stay together. Thus we must have a complicated Brain Area which monitors the couple's behaviour to insure it remains within the Allowable Bands as the relationship progresses.
The Mate Unfaithful Area is activated by the Mated Area as it is activated, and Mate Unfaithful will trigger Mate Reject if circumstances warrant.
It is noted however that Isobel Syndrome is triggered before a date has been offered when the girl thinks she should have got a date offer. We have thought of the Mated Area becoming activated over the first few dates as the girl begins to feel she is becoming the guy's girl (and Date Offer Inhibitor begins to be suppressed). During these first dates the rules probably are that you should not enquire too deeply what your date is doing (there may be commitments which are thought should be honoured) - but if one party is caught two timing it can be the end. Thus how these Areas are turned on, and how they turn on or off related areas is itself quite complicated.
A related problem is if a girl stands a guy up during these first few dates, and does not co-operate in rescheduling the date at that time. If the girl had enacted the date we assume she still would have waited to be asked for the next date - due to Date Offer Inhibitor. Therefore we can see that if she stood him up (especially if she went out with someone else when she was another guy's girl for the night) her Date Offer Inhibitor will make her wait to be asked out again by the stood-up guy. He however may assume it is the girl's job to co-operate in fixing firm arrangements - so he's not going to ask her out again but expects her to provide dates she can make for his original date offer - which she is not going to do. Thus she thinks he is not going to ask her out again (and if she would have accepted) causes her Mate Reject to be triggered with a form of Isobel Syndrome. Thus the damage the girl may have caused by standing him up is made many times worse - the guy may reasonably assume that she stood him up as she didn't want to go out with him - and her subsequent Isobel style hostility is to insure he does not ask her again. His Mate Reject is expected to be triggered - and they end up at loggerheads.