With Katie, it was odd. She had bfs and all that. Yet, there was this one gf she hung around with that became a FWB thing. Her name is Kassidy. One time Kas asks Katie to go a real date. So, Katie agrees. Katie said there was one point after dinner and in the movie before it started that she reached out to hold Kas hand. When they held each others hand, Katie said she was so comfortable and happy to be with her.
I knew something was a up a bit later on in their relationship. I was having a outdoor patio part. Of course Katie was invited and could bring a guest. She asked me to come to the front of the house. She asked it was ok if she brought Kas as her date. This was so out of character for her. She would just show up to our events with whoever and when ever. The fact that she asked if it was ok meant Kas was special. I was actually a bit disappointed she felt the need to ask. In the end, I introduced Katie and Kas as a couple and I used the term "significant other" when describing their relationship.
So, they are now married.