I agree, to the extent prostitution is a job it should be treated like one. I think whether it's right for them to service married people is like whether you'd knowingly sell alcohol to a minor. You as the seller might think it's wrong and enabling, or you might think it doesn't matter because you drank as a minor and you were fine, depending on your beliefs, or you might really need the money either way. Morality is secondhand to you being able to pay rent.
I think no one is beholden to keep anyone else happy. If you cheated with someone, it's your decision, and if someone married decided to cheat with you, it was their decision. If you refused, it was your decision. The SO might been unhappy, or they may not be, and you may be seen as a shitty person buy them or other people. But to all rights, they can't make the decision for you, you have to decide yourself.