In the first example, you are doing a disservice to both you and your partner.
In the second example, it is actually quite rare for a partner to not have an inkling that their partner is not being faithful. Some chose to turn a blind eye in the hope it will fizzle out as they want to stay in their relationship. But if it does happen that they were totally fooled, then only the partner has done wrong.
If we can't allow ourselves to trust, we can never have a real relationship.
RE: Did you ever purchase a prostitude ?
You don't get to choose.