You are a foolish man if you sleep in the same bed as a woman that you've yet to be sexually intimate with. 9 times out of 10 you will end up as her cuddle bitch who has been placed on the friends ladder (google "ladder theory" for more info) never to be placed on the "potential ladder."
What is your fear that you can't get with a woman that has no boyfriend? You're going to fall in love (if you haven't already) if you keep up this charade of a "friendship."
As for your questions: If I knew that my friend had romantic feelings for me when I was already in a relationship then I would tell him outright that I liked him as a friend only. YOU are the one that needs to quit playing with your own emotions by having this "emotional affair" with a woman that has promised herself to someone else. \\\
If you're confused then speak to her about how you're feeling and if she can't give you what you want then quit the crap and back off. You're wasting valuable emotions and dating time with her if she wants to keep the boyfriend until he's back to her home town or she marries him and moves to where he is.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion