I'm much older than you, married 25 years, and have been in a similar boat, even having seen things the way you do. And I believe before the internet and porn everyone masturbated to fantasies about people they knew, because, well, that's what they had.

The realization I finally had was that while I might be 100% sure what we're doing isn't cheating and isn't an issue and shouldn't be for anyone else, well... your girlfriend feels very differently. And who the hell are we to decide how they should feel?

Unfortunately, shit like this is not something anyone ever negotiates up front. So issues are common. Many guys just hide it...

The best you can do is try to have an honest talk and try to make each other understand... and agree to not do things she feels are cheating going forward. That's what we did. But, spoiler alert... there is always some new landmine to step on. ;)