What you (& your fiancé) are dealing with is possibly, IMO, so much more than a relationship issue. If he suffers from what's recognized as an addictive personality then it's no wonder he lies about quitting--then goes right on cheating.

I urge you to try very hard to look at this more objectively and consider that it's not a matter that simple choice can solve. You say, "I don't know if he would do this for me." Sunflower, if this is an addiction on his part, he cares far more about child porn than he cares about you.

I'm not advising that you pack up and leave, at least not without trying to get help for him. I would suggest that you ask him whether he ever feels overpowered by this need and let him know that support/treatment is available.

TBH, I will tell you though that recovery from this is low. We just don't know yet whether the pattern in illegal drug addiction are evident here, e.g., moving from use to distribution.

If he would not agree to getting individual therapy and couples counseling with you, I would then definitely end the engagement. You can't risk you son's mental and physical well-being. You might not be able to keep your son "off the market" of kiddy porn ... and sadly his photo may already be out there.

Please keep posting. You don't need to muddle through this without the company of others who can hear your rants and tears and everything else.

Roadie