You have to read between the lines.
Young women who report that their romantic partners look at porn frequently are less happy in their relationships than women partnered with guys who more often abstain, new research finds. Discovering explicit material on a partner's computer "made them feel like they were not good enough, like they could not measure up"
The key word here is report. In other words, women who DON'T report about their husbands porn use, or simply don't know about it, aren't having this same self-esteem problem. The ones who report it are having issues with it. It's a self-selecting bias.
Studies like that should be taken with a grain of salt because they are really hard to control and have loads (no pun intended) of lurking variables. Porn may be a relatively innocuous (or even helpful) part of a marriage or it could be really destructive, but there is always more to the story than just the porn. Porn is usually a symptom.
I agree with Samspade that for most people in small doses it is probably fine, but too much is destructive. If you are spending all your time looking at perfect young gorgeous women getting blasted from all angles, it fosters dissatisfaction, then guilt, and so on.