Exposing someone to the risk of an std without their knowledge and gaining consent to sex under the fraudulent pretense of monogamy seems like it should carry some weight behind it. This is assuming you continue a sexual relationship with the original partner.

It takes a level of sociopathy to hand waive cheating. That's not hyperbole. I'd rather be mugged than be cheated on especially if it was for an extended period of time in a relationship. That can amount to years of your life being fundamentally a lie.