Quote from Cutten:
You are forgetting his wife. His marriage vows prohibit infidelity - that's breach of contract. Affairs cause immense emotional distress to the wronged party - not sure if that's illegal, but it's certainly immoral and cruel. Then there's the very real chance that by screwing a woman in a high-risk group, he will contract an STD and then pass it on to his unsuspecting wife - that is reckless endangerment of someone's health, and if it isn't illegal in the US then it damn well ought to be.
How would you like it if you were married, and your partner screwed a hooker then gave you herpes, genital warts, hepatitis, syphillis or even HIV?