I KNOW HOW YOU FEEL! I was actually with one of my friends for a while as 'fwb's' but then I ended u moving on onto an actual relationship and he wasnt mad at me, but when I turned back to talk to him again because we ignored each other and he got attached to Luara. I was devastated.
I guess teh best way to handle it, is to just talk normally and let things take its course, because of things were going fine before, then they should keep being that way. Obviously you were talking to him liek this and dintknow about his girlfried so you were soing perfetly fine. Thats how it was for us but when i turned teh corener for a second and came back everything had changed. Just dont lose contact with him
And to Neche_Girl8, thanks you very much anyways :)