Is it because cheating is a fundamental breach of trust? Or is it a deep-seated fear that cheating may lead to the risk of dissolution of the relationship and the man directing their resources elsewhere?

I've also heard explanations that say for some women they equate the act of sex to love. So if the couple stops having sex or the man has sex with other women it means that they don't love her anymore.

Even if one tries to have open honest conversations about this, why is it hard for women to understand that it's possible for men to be deeply in love in a LTR and yet want to have casual sex with other women?