<human>: My SO is currently in South America on vacation and I'm staying with my best friend of 13+ years in his studio apartment, taking advantage of some down time before I start grad school. Mostly we have been on our computers just playing games, watching movies, etc. Until at one point, my friend, in a fit of boredom, puts on some porn.

I saw he was getting into it, but that he didn't want to just start jacking it in front of me. I figured "Hey! Im a sex positive person," and told him not to mind me and jerk off if he wanted. He did.  

After giving it a couple of minutes of watching the same porn I figured "Eh! What the hell, when in Rome." Masturbated myself, came in about 15 seconds and went back to computer stuff. We did not touch each other, did not make eye contact, did not even make noise. After cumming, I even found it comical.

Anyways, the very next day I tell my girlfriend about the experience thinking she might find it funny like me or even erotic. She reacts with confusion, uncertainty and ultimately asserts that she considers this cheating (which I didn't even think about). She takes issue with the fact I was thoughtless. She said she's unsure if she can ever forgive me or trust me again.

And to me. . . it was just a wank. There was no thrill associated with infidelity or a sexual bond with another person.  I suppose just want to hear what Reddit thinks about this (especially any women so I can get my GF's perspective). Did I cheat? Is there any way I can explain this better to my Girlfriend? Any other advice to help me empathize with my GF.

And just to answer some complicating factors. I identify as heterosexual (this is the most homosexual thing I've ever done) and my friend identifies as a heterosexual. Furthermore, me and my girlfriend are liberal and atheist, so we do not subscribe to any rigid or conservative ethical doctrines.

Can you succinctly summarize the text in 1-3 sentences?
<bot>: Masturbated to porn at the same time as a heterosexual friend in the same room. Girlfriend says I cheated, doesn't feel like I did. What do?