However as you say most people think it's wrong so are we already I t he track of saying that in reality the moral judgment is already there?
Yes, of course it is. Our society is set up around monogamy, it's illegal to have more than one spouse and cheating is a perfectly acceptable reason to end a relationship and legal grounds for divorce. In fact, many people in unhappy marriages often say they wish their spouse would cheat so they'd have an acceptable reason to end it. If anything, merely being unhappy isn't considered an acceptable reason. The "exit affair" is a thing: when an unhappy spouse has an affair just to provide the justification of ending the marriage.
It hint he idea of contractual obligations in a relationship is an interesting debate but some do not get married but surely those involved in cohabitation including children deserve to have at least an expectation that fidelity is maintained?
That's between them. Not everyone chooses a monogamous life. If you want to enshrine it in law so that there are legal consequences to breaking the promise, you can as long as you both agree. You can end the relationship if they cheat or for any other reason.
if we do have HSE in schools and part of the curriculum is to define what is important and healthy in a commuter relationship should we just reiterate that infidelity is definitely a barrier to this?
I don't think it's the place of schools to tell people how to live. They should teach about safe and consensual sex and being prepared for the intensity of emotions that it brings. At some point, life is going to have to take over as teacher.
You seem to think that women don't feel justified in ending relationships due to cheating, which I find odd for all the reasons given in the first paragraph. Infidelity is a perfectly acceptable reason, socially and legally, to end a relationship. People often engineering it because they can't envisage ending it for any other reason.
In my experience, women who won't end relationships when they really should after cheating do it not because they don't feel entitled to, and not even because of the kids, but because they fear losing their lifestyle/poverty and all the upheaval and change. That's a very understandable reason, especially these days, and I am not judging. But it's not because of a society-led acceptance of cheating and additional opprobrium won't help.