Are you feeling attached, confused, agitated, or in love, etc., with this friends with benefits?
Come to terms with your mixed feelings.
This is how you isolate your feelings and detect which ones are strongest.
You have to be completely clear about them.
Feeling a stronger sense of attachment than you think it warrants in a friends with benefits relationship reveals that you are on the path of falling in love with them.
Not mutually deciding to date each other exclusively, frees up the girl you are dating, as she does have the option to date multiple men.
Remember that until the both of you decide to become exclusive in your relationship, she has the right to date whoever and as many people as she chooses.
Figure out what you want out of this relationship.