You're looking for the word "sue", OP. Fined makes it sound like the government is peeping into the bedroom.
Anyway I can see the case for it. If it's implied in a marital contract that you wouldn't cheat, and you do cheat, then that's breaking the contract. What kind of penalties that should be associated with it though I'm not sure.
I heard this used to be a law in Taiwan, but the wife would usually be magnanimous and not sue a cheating husband, but husbands were a lot more likely to take revenge on cheating wives and so the law disproportionately impacted women.