Until after marriage maybe out of shame or just thinking we know best. We’ve almost been married a year, just welcomed our third child and it’s definitely better
A lot of guys can struggle with porn addictions. There can be a way out of it if he sees it as a problem and wants to work on it. It can be such a strong pull but he could realise he will feel better without it. Its like dopamine chasing. Marrying in the catholic church is forever. You should only do it when you are sure. But having a child together is serious, i hope you find a way to get through it. Perhaps looking at both of your attachment styles could be helpful. Being securely attached is a good basis for building a strong, trusting relationship, and following catholic guidance can help not be tempted by sin.
Also try and start a routine of prayer together!
I would speak to a priest about it, marriage is consent from both parts and right now it doesn’t seem like that’s what you want to enter. I’d be honest with the priest, I was in a similar situation and just ignored certain things, got married but it only seemed to exasperate problems even more. It wasn’t until we both spoke to a priest together continuously and I was completely honest about where I was at and how I was feeling that we really got somewhere. Not getting married just yet doesn’t mean ending the relationship but also remember you’re not supposed to go into marriage as perfect beings. My husband actually slept downstairs on the floor for a year before we got married as we didn’t have the space and wanted to make sure we were living together as brother and sister in Christ (which was already hard as we had 2 children). Although we were doing what was right, we neglected eachother emotionally and that caused problems, we had guidance as individuals but didn’t seek guidance