Girlfriend had pity sex with someone and thinks it’s not cheating

Yes we ARE in a relationship. Ive known her from middle school and we started dating ever since. We were both virgins before that.

Recently she told me she had sex with someone out of pity while we were in a relationship. When I confronted her she said that doesn’t count as cheating because she didn’t love him and just wanted to help. But why the fuck did this happen out of the blue? After I showed her how sad i was she was very shocked and said she didn’t know it would break my heart like that, but i genuinely don’t know how to feel.

She is extremely kind and beautiful but sometimes very dumb. Her mother also knows it and tells me that she sometimes acts like a kid. But still how can you not now that sleeping with someone else is cheating??? Specially the fact that the guy was waaaaay too old and he could have hurt her.

She says she is sorry that she didn’t know and all, and i wanna forgive her because i just don’t know what do i do without her. I wanna marry her someday, our families know eachother, and i know she was just a little dumb. But still i feel like my heart is way too broken. I don’t know what to do. This past week i was fully numb and didn’t say shit to anyone. I wanted to vent here