That's part of it dear.
If you and the guy don't have any obligations to each other, that extends to any obligation he has towards you as far as whoever else he has sex with.
If you have no obligations to each other, your friend is free to do whatever she wants.
If you liked to play golf, and someone who was an equally matched player agreed to play with you on certain days, would you be upset if your friend arranged to play with him on days you weren't?
I can hear you saying "but that's different!" Well, no, it isn't. Not really. However, we women do have the annoying habit of bonding through sex.
Yes, I am a woman, and all the women who have anwered you, including me, understand how you're feeling.
We are hardwired to want a stable relationship with someone, especially when we are younger. That's another subject.
I'm not saying this faulting you, because, especially at your age, I'd feel the same way.
You want a FWB.....but only as far as you being the one that gets the benefit part.
You don't have to do all the work of maintaining the emotional part of the relationship, but you do have to do the work of accepting that makes both of you free agents.
Like jes said, you can't have it both ways....that doesn't mean we don't understand.