Men are less selective the less they invest into a liaison. It may be a matter of convenience or simple variety. It may be that she has a worshipful crush on him already and feeds his ego in a way that he enjoys.As such, the woman he has cheated on you — to simplify — may be fatter, uglier, of a bad reputation, much more dubious character or even older.This leaves you feeling quite disgusted. If he cheats on you, at least she should be seen to be worth it in some way. It also decreases the perception of how attractive he is to your eyes (“really, couldn’t he do any better than that piece of trash?â€)His cheating with anyone who you perceive as less attractive than you are decreases his worth on a visceral level to you.
On a social level and if his cheating incident is widely publicised, this greatly reduces your perceived social value to those around you.
All of these factors may cause you to have to end it with him: remaining with him makes you unable to respect yourself, respect him and those around you unable to respect either of you, if they know.