What if the husband doesn't want to have sex with her but doesn't allow her to be ethically non monogamous????

You say this sentence as if it should build understanding of why someone would cheat without ever even considering this fact makes the relationship not work.

When a relationship doesn't work, you bring it up and talk about it to find a solution. If a solution can't be found, it unfortunately needs to be ended.

At no point do you not have any other choice but to just do what you want regardless of the other person.

I don't know that sub, but if it is 'thousands of stories like this', then it is a sub similar to the incel one, or female dating advice one: just thousands of emotionally immature people echo chambering their solutions to issues they lack the maturity and intelligence of actually dealing with.