You are taking an extremely passive stance, and are putting a lot of onus on others to read your unsaid thoughts and feelings, this is a recipe for disaster.

&gt;Imagine meeting someone, forming a connection with them (sexual and friendly), then them meeting someone else (worse, your own friend) and basically making you a third wheel.

You made yourself a third wheel by not expressing how you felt to the guy in question or to your friend.  He is under the impression that this is a FWB situation while clearly to you it isn't.

This is not about respect at all, you haven't set any boundaries with anyone.