When I have decisions like this to make, I make sure I don't make my decision based solely on fear. Fear is a helpful emotion and I listen to it but it does not drive the bus. It gets to have a respectful input but no more.
So in this situation, I would ask myself exactly what I fear. For example - Why do I fear sex will lead to the end of the current friendship? How do I think the friendship might change if we get together? Don't get together? Do I think that sex changes everything and one can't be friends with sexual partners if things go awry? What could get ugly, if anything?
Once I know why I am afraid, then I can figure out if I can live with the potential consequences - unforeseen or not. (Sometimes potential consequences are good things!) If I can live with the consequences to the best of my knowledge at the time I made the decision, then I go for it (whatever it might be - grad school, new lover, career change, moving).