Picking up on three things you've said here.
1)sex is infrequent and always in the same position
2)when you separated for a short while he had sex with 2 other women
3)he prefers wanking to porn than sex with you
Ok, so I'd be thinking here is a man who does have a sex drive, is not completely inexperienced, but has no imagination. He doesn't change things up or experiment. He needs to watch porn because he has no imagination. And his preference is for.......change, novelty. But his need for novelty combined with his lack of imagination leads him to tire of one woman, novelty for him is about different women.
This points to several things, a lack of emotional maturity, lack of empathy and emotional connection towards the person he has sex with, an inability to see sex within a long standing relationship as a means to show love and receive love.
I'm inclined to think that he needs therapy, but not to exclusively deal with porn addiction. But then I'm also inclined to think that life is too short to have crap sex with a man who isn't even in touch with himself.......let alone trying to find ways of pleasing me.
It's interesting how many ways an avoidant personality can find to prevent anyone ever getting too close to them. And the psychology of someone whose desire for novelty only exists in having sex with people they have no emotional connection with, suggests that in some cases this is also about his relationship with his mother.
So, what is his relationship history?