I am answering the first version of this question.
Nothing is inherently wrong under a utilitarian framework. Utilitarian judgments are contingent judgments.
You have omitted in the utilitarian calculus the negative experience of the person who is disposed to prefer exclusive partnerships nonetheless being in a non-exclusive relationship. This will not be a good time for them.
But if you are proposing simply changing one's disposition, you also omit the alternative of just eliminating infidelity within exclusive relationships.
I will not continue to edit in response to a changing question, but one of the new questions youYou also ask is "why is exclusivity so highly valued in the first place?" That is a question about psychology, sociology, biology, and evolution.