Very few human beings are completely fine. Most have weaknesses, childhood shit, insecurities, general flaws.
Marriages, likewise, are not bomb proof. It's work, there are ups and downs and we can really love our spouse but still have all sorts of things going on.
So imagine I got dressed up right now and headed to my local pub. Let's say I find one of the school dads and I flirt with him.
Let's say I laugh at his jokes. Let's say I hold eye contact a bit too long. Let's say I flirt, and make myself appealing.
Sure, there's a chunk of men who wouldn't feel anything. There's also a chunk of men who'd instantly be texting me. But most men are human and would fall somewhere in the middle.
They might start wondering what it might feel like to go to bed with me but dismiss the thought quite quickly. They might even be a bit stiff with me or avoid me.
But let's say we're thrown together unavoidably. Like I get a job where we have to work closely together. And Let's say I give him lots of attention, maybe drifting into emotional support.
He loves his wife, so he will tell himself its no problem. You can be friends with the opposite sex, right? And he decides he's a good guy so it's fine. Be mates.
Let's say I then approach him again on a work away day, again, lots of deep eye contact and flirting. And despite himself he finds himself getting turned on and wondering what it might be like to take me to bed.
At this point...the man is already probably fantasising about me in the shower, and he has a bond with me on an emotional level. Let's say this makes him a bit less interested in his wife - because that feeling, those endorphins feel quite electric.
He loves his wife. He doesn't want an affair. But now he can't stop thinking about me - about how I make him feel. The way I listen to him and hang on his every word. The way we're such good friends. The way I pay him all that attention while his wife is (as a PP said, at home, frazzled and dealing with laundry).
At this point infidelity has happened already and where it goes from there sort of doesn't matter as the marriage is now in peril of a sort.
He's not a "bad man. He isn't in an unhappy marriage. He's just a human being with all the flaws of most responding to a biological and psychological dance of courtship.
If he's a man with great emotional skills and high self esteem, he might ask for a job transfer or otherwise take steps to nip it in the bud.
If he's not (and many people aren't) factors like low self esteem, poor coping mechanisms and so on might lead him to bad choices that might really devastate an otherwise happy home.
This is quite often the anatomy of an affair. Two people who engage in bad choices because something feels nice and they continue with it.
The married person is obviously responsible for the betrayal and their own decisions. But as my OP said, if the OW knows the man is married and has gone after him, then she's also got a portion of blame here.
Marriage isn't meant to be a gauntlet. You're not supposed to see your spouse off in the morning while they valiantly fight off people trying to shag them like space invaders.
It's simply meant to be the case that if people are married - you leave them alone and set your sights on available partners for yourself.
As others have said, it is just not that hard, and if we choose knowingly not to do that, then we are also responsible.