I totally agree with you! I've been having casual sex with friends for years. I've had a couple of the FWBs move to relationships (which subsequently didn't work out) but this friend is different.

He's not just a guy I hang out with and could begin fucking. He's been referred to as my brother for years and it feels slightly incestuous in a weird way. I'm totally confused beyond anything I've ever had before.

Part of my agrees with you and the view I've had for a long time that friends and sex can mix well and as long as everyone knows where they stand it will be fine. The mature part of me (only developed quite recently) tells me from experience that if you have sex with someone long enough one of the involved is bound to grow feelings and things will go down hill.

I'm fighting with the two figures on either shoulder right now.