In this situation you need to make your decision based on your feelings alone. Don't let people influence you based on how they think you should feel, in the end you're the one who will feel whatever. You have to be 100% sure in whatever you decide. If you choose to fully get back with her you must let go of what she did (any mistrust, pain, anger, etc), all the emotions must be let go on your part.
If you decide that you can't do that then you don't need to force yourself to fall back in love with her (which is already a bad sign because that is supposed to be natural). Holding on to any negative emotions you felt from her actions will eat away at your mind and heart and eventually cause the relationship to deteriorate anyway. So, please take the time to think long and hard and ask yourself these kinds of questions.
Do It For You
Also, if you decide to take her back you need to ignore all the comments of those who may disagree with you. It's a lot. Good luck.
I'm not a fan of the whole "sleeping with another person to get even" thing. That would just bring in more drama. Getting back at her should not be a goal at all, if you are thinking of taking her back and giving your all to her again. I think you might think it is a fix, but your girlfriend is just suggesting this to ease her guilt, essentially, she can then just say 'you did it too'.
Keep it simple, if you love her and still see a future with her, get back together with her and do everything you can to make it work. If you think you would be second guessing where she goes, or checking her phone all the time because you don't trust her, then I would suggest a clean break up now.