<human>: We've been dating for well over a year know and we were just talking back and forth tonight like usual. Suddenly the topic changed and we we got to a conversation about finding other guys and girls cute. 

One thing led to another and somehow masturbating got brought up and she was talking about how she had masturbated to guys before we dated but worded it as if she was still masturbating to guys know. 
I wouldn't have minded because on occasion I masturbate to other girls. I felt it was just human nature to do so and didn't take it too seriously in finding harm in it but she didn't feel the same. 

I talked about general insecurities and that she shouldn't feel bad. All she could talk to me about was though that it's because I'm no longer "sexually attracted" to her so I have to find girls attractive to please myself to. I know that's not the case at all because I find her very attractive still. She also kept mentioning a "weight gain" (non existent if I might add) and then began binge apologizing. I didn't know what to do so I kept apologizing too in hopes she'd feel slightly better. Nope.
  
I don't think she grasps the idea of it not being about a loss of attraction more so just a fantasy I could care less about when I finish. 

It's hard to elaborate but I feel like most guys, and honestly girls, have masturbated to other besides their significant others and their relationships haven't been strained because of that fact.

What can I do or say to help alleviate the problem and let her know this has nothing to do with her or a loss of attraction?

Condense the text into a few key points
<bot>: Girlfriend found out I've masturbated to other girls while we've dated. Got very upset. She's feeling insecure. What can I do to fix things?