Most Helpful Girl
Because the betrayal and loss of trust invovled in cheating shows how little your partner actually cares for/respects you. Even if my boyfriend only cheated on me once, I wouldn't be able to view him as my 'boyfriend', or someone who I consider myself in a committed/exclusive relationship with after that. He would become nothing but a fuck-buddy to me after that, regardless of him being down to fuck around the clock.
Most Helpful Guy
Let's define cheating. Imagine two people who are friends but sleep with other random people all the time (fwb). They have no commitment or expectation of being loyal to each other so they don't risk or lose anything by sleeping around. With cheating you are betraying the trust/agreement you have with your partner. You're risking your relationship and you could lose it. Why does cheating matter? Because it affects your partner in ways you promised not to. IF your partner says you can have sex on the side it's not cheating