Firstly, having sex with another person while in a committed relationship is not fundamentally wrong. It might be a breach of the agreement the partners have implicitly or explicitly, but there's no compelling reason for an agreement for monogamy exist besides culture.

Moreover, when two people have consensual sex, it is because they want to and accept responsibility for the activity. Unless you mean to say that everyone who cheats is being raped, you need to come to terms with the fact that a woman (or man, or anyone) can willingly have sex outside of their relationship.

It is possible that one regrets having an affair, but that doesn't mean they weren't willingly entering it. We can buy books or movies or games or other things that are very appealing at the time and that we realize later we don't actually like very much -- but that's still our fault, not the products. It may have deceived us, but lies and deception have been a part of human life (and animal life, actually) for millennia, and the only recourse to being swindled is to be vigilant of being misled.

To justify your belief that responsibility for an affair is on the outside party, you've allowed yourself to believe that it happens because the feelings she has for her partner are conflated into her extra relationship. It is entirely possible for a person to simultaneously feel any emotion towards multiple people separately, even love or attraction. The adulterer is not stealing these feelings; love cannot be stolen. They are, through honest or dishonest means, earning those feelings.

Applying the same reasoning to hypnosis, that responsibility for a breach of an exclusive hypnotic relationship is not on the partners in that relationship, but instead the result of some unfortunate and exploitable mental weakness is unhealthy.

When you allow yourself to believe this, and you communicate this concept of the mind to your subject, you are telling them that their mind is weak, that they shouldn't engage in hypnosis with anyone other than you, that they need to learn to defend their mind from other hypnotic controllers, that they are not necessarily responsible for their own hypnosis, and that they can be easily led to do something against their will. That they are an easy victim.

This is textbook emotional abuse. Instead of developing in your partner a healthy relationship with hypnosis, you are telling them that it is a powerful and dangerous force that they are subject to. You are telling them that they are not in control. You are also telling them that it is wrong to be promiscuous, which it isn't: a healthy person can engage in consensual hypnosis (and sex) with whomever they want! They have no moral obligation not to decide that they no longer want to be in an exclusive relationship.

I don't know your exact situation, though I have my own theories, but I would like to give you a word of advice that I tell myself: You do have sexist beliefs; you do have prejudices; you do have delusions; you do have (self-)abusive behaviors. The only solution is to think carefully about how you think, and study the things you are afraid will prove you wrong.

I hope that you can find a way to make your practice into something that is truly good for you and your subjects. This post will almost certainly offend you, but I hope that it ultimately proves helpful in someway to solving this problem you're having with your practice and relationships.