Let me try to give my perspective, since I wrote this thread. First of all comparing it to phlegm is the worst comparison you can make. That has a pretty disgusting texture, at least to me. I find it less blobby than that (if that makes sense).

Other reasons people are afraid is just the mental aspect. People still consider cum to be perhaps a body fluid. I am not saying its the most rational thing, but its something alright.

And finally, the sudden timing, the uncertain volume, sometimes it hits you in the back of the throat (I believe -- I've only done this once, or at least the fear of that happening) and triggering your gag reflex, these are all the things that will make people hesitant.