I think there are many naive people around. I was also in that camp myself. I never, for one moment, suspected that my "D"H could cheat on me. Or that he would even want to. We had been together for 16 years, when I found out that he had cheated with several OW. We had the PERFECT life. Two lovely children, plenty of money, good sex life, big house, 2 cars, foreign holidays, and back then, I was really pretty with a lovely figure (not any more, LOL). BUT....he still did it anyway. And not in a passive "I was caught in a situation and was weak" kind of way - he actively pursued OW's.
I stayed for 4 years, trying to save the marriage. He continued with the sleazy behaviour (which I was far more aware of, once I knew to look for it), and guess what, one night, in a bar, I was approached by a handsome man, and BAM, I cheated on my "D"H. I would have NEVER done this, had he not broken us first, but by this point, I was just so heartbroken. So, there you go - I am that someone, who thought I would never cheat, but given the perfect storm, I did.
This was now all years ago, and over that time, all of my friends husbands have cheated on them. Even the quiet ones, that you would swear would not have it in them. Every. Single. One.
I think when you're in your 20's & 30's, you haven't seen much of life yet, and you trust everyone. There is that innocence, inside you still. Sadly, once you are older (I'm in my 50's), you've seen so many things happen, that you get a bit jaded. I suspect that's why OLD in your 50's is a shit show. No one trusts anyone by this point, because most people have been shat on.
I see my daughter and her friends planning their weddings, and feel dread for the heartache that could come, if things pan out they way they did for me and my friends.
But....you can't live like this, can you? I married again. I trusted again. Thankfully, at 15 years in, things are still good, with no signs of foul play. But I would never rule out anything. I'm happy, but vigilant.