See, that's what I mean - "cheating mentally?" That's nonsense. And beyond that, it's certainly not the same as cheating physically.
My husband loves me and respects me, and if he spanks it to porn when I'm not around, great! Porn isn't a threat to our marriage, and it never has been because neither of us makes it a big deal. Hell, sometimes if we're in the mood we watch porn *together*. I've been with partners who have cheated on me physically, so I know what that's like, and a little bit of porn when we're not together is a non-issue. It is a fantasy, and as long as it has no discernible negative impact on your function as a couple, then I can't fathom the problem.
If he were cybering or camming, I can totally see being justified in your concern. The internet makes those boundaries blurry sometimes, but I think cybering and camming could, under most people's definitions, be considered cheating. But jerking off? Get over it. He jerked off before you were on the scene, and he will jerk off when you're gone. To ask a man to stop jerking off while he's with you is, in my opinion, silly, controlling, and rooted in insecurity.
The reason I'm saying I think it's controlling is because you're trying to police your boyfriend's thoughts. By your own admissions, you don't even want him to *think* about another man sexually, lest he be guilty of "mentally cheating." I call that controlling.
You don't want him to be able to enjoy his own body *by himself* - a body which is his and not yours. You are policing his body as well as his thoughts, and I call that controlling.
Would it be different if he masturbated in the shower? He could be imagining you, or he could be imagining getting gang-banged by the lacrosse team and you'd never know. But as long as he lied and told you it was you he was spanking it to, would that be okay?