[Massdebator]

If she wants to have sex with someone other than you and won't tolerate you doing the same, then yes, she is being selfish. It's called a double standard, and is very similar to, "Do as I say and not as I do." Though, if the two of you are not in an exclusive relationship she can't really stop you from exploring other options yourself. However, I had a hard time understanding exactly what kind of scenario it was that you were laying out in the text because of the way you worded your message.

If you haven't already, talk with her about the issue at a time appropriate for serious discussion, and be civil about it. Ultimately, you two have to decide if your relationship is exclusive or not, and in the event that she is inflexible in her stance that she has the right to fool around and you do not, the two of you will need to decide if that is a dealbreaker. If she is not willing to have a threesome, perhaps she will be open to the idea of you having sex with someone else when she is not present. It could just be that she doesn't want to be there when you do it.

As a side note, it's important to know what constitutes cheating. Cheating is a term that really only applies to marriage and going outside of said marriage. If your girlfriend were to go outside of your relationship and fool around with another girl, she would not be cheating. All that means is that your relationship is no longer exclusive.

Cheers!