My problem with my girlfriend.

Well i've been dating this girl for just over a year. but before i even met her i was in a relationship with someone else who i had sex with a few times. MY new Girlfriend has only ever done anything with me (except kissing and feeling bums). she is feeling weird because i've had sex with another and she hasn't, she has decided that she needs to have sex with someone to even the score so she doesnt need to think about it anymore. Now the person she is going to sleep with is someone she has spoken to for year over the internet, she has never actually met him, she is going to have sex with him at T in the Park. which they are both going to. i dont feel it will fix anything, i just feel myself ready to break down every once and a while when i think of it. Don't get me wrong, i am willing to let her do it for the sakw of keeping her, but i just don't know if i will be able to readjust.
-Extra Information
My previous relationship wasn't that great, i got cheated on, and i kept going back, leaving me a little uneasy, so i know that i dont like the fact of my girlfriend sleeping with other people.
My new girlfriend likes to go out and just recently kissed someone else, i feel like as she has now cheated on me it should be a deal breaker and she should be content with we now have something to think about.
Has anyone felt like this?