it's simply in their nature for men to cheat on their partner.

Yes. Indeed. Most likely. Not exactly cheating but polygamy. Which is demonstrated by the fact that most of the men that have many sexual opportunities with a lot of different women (very attractive and/or very successful men like singers, actors or sportsmen) end up with a lot of different long term and sporadic partners and several children across the globe, lots of them unknown.

So probably, if an average man has enough opportunities, he will try to spread his genes. Even more, since the last decades, humankind has contraceptives and safe abortions so we can behave as we want sexually reducing the risk of facing the natural consequences of sexuality.

Look at all these anecdotes of men cheating on their girlfriends written here

Lots of them are probably true. Men cheat a lot also.

But cheating is more complex. You need to be willing to break the internal contract or code you've arranged with your partner..so heavy morality comes into play.

Cheating on a gf is not the only phenomenon driven by male promiscuity (and is not only driven by promiscuity, other factors intervene), also leave a partner to be with another, polygamy, etc.

Adopting this mindset is self-defeating

You have to be realistic. A lot of people, men and women, don't want to control their natural impulses. Especially if you can't identify them, you can't control them.

I am going to identify bad behaviors early on and avoid men I think will cheat on me

Good for you. Well done.

and one you really do have to prove

You also have to prove that these 'toxic ideas about masculinity' are the cause for this phenomenon that men are constantly reporting. The majority of these events happen after the girl insists on her partner to 'open up'.

I will repeat it. It's not about ideas. It's the same kind of process that drives a girl to feel more attracted at first stage to a tall guy more than to a short guy.

they stop looking for exceptions

Oh, exceptions sure do exist. A lot of them in fact. There are approximately 3 billion adult women in the world. We all would be doomed if exceptions didn't exist.

I do not want them to accept it as normal.

Don't accept it if you don't want. But these unending parallel negative events that all men seem to experience with the same patter, I can give a more logical explanation and with more common sense that the one you're trying to give. It's a matter of triggering the mechanisms of sexual and romantic attraction wired in the brain. Sure if you combine it with the preconceived ideas abou gender roles, it just became more powerful, but that's not the base thing.