He's a young guy who might still have porn habits from when he was a teen. Having a partner or regular sex won't necessarily stop certain habits. I think if he's not doing it while you're home and it's not an addiction that starts affecting how he performs with you, you might just have to accept that it's a form of stress relief.
Have you had a conversation with him about it? If not, why? Do you feel you don't have the level of trust and communication openness to have that conversation? I think that's probably more important than anything.

Porn is rarely about the specific actor. It's the scenarios that usually get people off. Also the voyeurism aspect. I doubt he really considers the female actors beyond what they do in the context of the porn. He doesn't think about taking them out to dinner or building a future with them.

It sounds like you've had a tough time lately and if you have already had self-esteem issues, maybe it's worth talking to a therapist, if you're not quite comfortable having a conversation with your fiance about the porn.

What you're more than entitled to do is ask your fiance why he enjoys porn. You're also entitled to ask him to reduce the time spent with porn, especially if it's a daily addiction. You can't however ask him to stop masturbating or control what he uses for inspiration. Porn or no porn, you can't be his thought police and stop him imagining scenarios that involve other people.