There really is some utter shite being spouted on this thread.
Was coming on to say pretty much what @shaggedthruahedgebackwards said.
Most affairs don't spring up out of nowhere. They're usually the result of two people being thrown together, and getting to know each other over a period of time.
The way people used to get together before tinder et al.
It's very simplistic to say that it's because the women has low self-esteem / a low moral compass - clearly that is sometimes the case, but such women also end up in relationships with single men, and many single men are total arseholes, as well.
Some of the ideas being punted up on this thread are total cringe - single men are raw? Married men have been 'vetted' by someone? Woman want an 'alpha male'? Are motivated primarily by a man's fertility?! If this were the case - everyone would be in relationships with unavailable people.
Where do these cod psychology ideas come from? Cosmo?
For the most part, affairs are the result of two people spending time together, liking each other, and not having the cop on / wherewithal / willingness to recognise what is happening, and address it.