I do believe that women are very patient when it comes to handling relationships; hence, when a woman totally loses respect for a man, it must be due to a multitude of reasons. During courting, men always put their best foot forward. It's as if they are willing to give everything just to show the woman how much he likes her. But the moment this courtship turns into an actual relationship, the other foot follows. This foot carries with it the true self, along with other unlikeable traits. I'm not saying that this is true for all relationships, though. When you give a woman your time, she values it most and hopes that you will be the same as the relationship progress. Hence, I do think that one of the reasons a woman loses respect is lack of attention for she will think that she isn't part of your priorities.
Moreover, and I think one of the major reasons is being cheated on. This is very painful even for a man. Knowing that you have been pouring your love to the person you value most but on the other end, you've been sharing his love to another woman. Being cheated on also makes a woman doubt herself because this would mean that she has never been enough to deserve his love. Hence, I do think that cheating is never forgivable.