When a girl brings up the FWB thing and you want to take things further. Right then you have to talk to her about it. When it's clearly HER wanting something from YOU.
I'd just tell her it's not like that, that she's a great girl, etc. Just don't try to commit or anything right then. And maybe start being a little more serious with her. Ask her for her opinion on stuff, plan more things together.
You always want to take advantage of your freedom and lack of commitment. Your freedom has to exist and be a thing in her mind. She has to see you are going out and having fun with women away from her. Like this she is going to want all that attention for herself eventually. Trust me a day will come when you tell her you are booked for Saturday with a group of guys and girls and she's not one of them. When she tells you she doesn't want you to be around other women. You have to play the jealousy card a bit on most girls. Women really understand jealousy...
Speak her language...
Just don't pass it with her, there are very fine lines between jealousy and emotional destruction. You have to give it to her in a way she can deal with it. You're with other women that are showing interest in you...
