Your take is that all married men cheat, it's normal. Did he tell you that? I know he told you that "she means nothing, it's just sex".
You give a person a piece of string, and they will take a yard.. You've allowed this woman into your backyard under some silly belief that he has put into your head. You were already told something that I bet you didn't even think of, in another post. Your husband is lending his clothes to her son... Sex only?
I don't think she is a fool. She was told by him "not" to go to his job. But she did. And, what did he do? Have sex with her and you are asking why he did that? She is no fool... She knew she could turn up and instead of her kicking her out, he'd have sex with her.
There is but one fool...