What if you masturbated after reading a bodice-ripper, would that be cheating?
You can't have it both ways -- the problem isn't what he did, it is that it was secret AND the problem is what he did. So, which is it?
If you are going to allow H space to use porn, how is this going to work for you when you consider his use of porn to be cheating?
Would you like to watch porn yourself, either with H or alone?
What prevents you from enjoying porn? Are you afraid of enjoying porn?
Do you really want your H to feel bad about having kinky fantasies and disgusting for masturbating? That isn't going to make him feel very good in general...
Are you afraid of being OK with porn and enjoying it with H?