It is something I'm personally affected by but I'm sure I'm not the only one so I'm posting it more as a general question.

Suppose that you are in a relatively (and intended) long term relationship with someone. You both seem to have the same values about monogamy. Then, after a year or so your partner starts acting oddly; withdraws, no communication for weeks etc. You get suspicious, confront him and he admits that he is married. He insists that he doesn't love the wife, he is not in a relationship with her any more, etc.

You are shocked and ask him how come he never mentioned this little detail. He says "you never asked".

I think I am NOT some weirdo believing that being single means being single both in effect and legally.

I know I would never, ever go anywhere near a married man. I think concealing this "little" detail indicates a complete lack of integrity. I think it is a conscious act of cheating.