Technically he didn't cheat on her since he was with the older woman before FL and that he told FL that it's only a contract marriage and that he has another woman.
That said, I don't like him either.
He didn't cheat on her
He did, whatever the reason they were married
That's not cheating. It was an arrangement to be fake partners. If you wanted to point out him cheating on anyone, it would've been on the first lady since he was in an actual relationship with her first.
The story now is cute but the fact he cheated on her is always on my mind so I'm unable to like this manga fully. So the FL is a virgin and the ML is a experienced man