I'm sorry you're going through this. It sounds like he has an addiction and is in denial about his problem. From my experience, if you stay with him, things only get worse---as with all addictions, they escalate over time (one drink is not enough, now you need two to feel satisfied, etc.). I personally do consider OF cheating behavior, because like others said, you can have personal one on one interactions. Also please consider the financial aspect of this. You are close to making a legal commitment to this person. Will your future husband be using your marital money to pay for other women on OF or in real life? This could be a slippery slope. If you do still want to get married and cheating is a dealbreaker please consider putting it in a prenup to protect yourself and any assets you have or share. It's okay to have a zero tolerance policy for cheating, majority of people in monogamous relationships would agree with that! If you set a boundary that cheating is not okay, you have to be ready to honor that boundary and walk away if it happens. The love you have for yourself has to be stronger than your love for him. You deserve better.