Posters have pointed out that masturbation doesn't have to involve porn use and that's very true .. and without access to porn men would (and do) masturbate without it. (Same applies to the percentage of women who use porn).
Problem is; it's there, it's available, it's fast, convenient, effective, easier and lazier than using your imagination, it's a habit - it's the (all ethics aside) the ready-meal equivalent of food for masturbation. And on top of that, given the opportunity, it seems that many men will always choose visual aids to masturbate.
(Some women too, plus studies have shown the cliche that women are less responsive to visuals is false, they actually respond to a wider range than men, but that's a whole other tangent).
You can certainly speak to your partner and say you wish they wouldn't use porn to masturbate, but unfortunately the likelihood is quite high just hide it. Also op has a problem with her partner looking at naked/risque images of actresses or whoever he fancies; that's not really porn. If a woman was on here saying partner was angry & hurt at her looking up body pics or sex scenes with Jason Momoa or Aidan Turner or whoever, most posters would think he was deeply insecure, controlling and unreasonable.