I feel the same way about cum that isn’t directly from the “tap.” Somehow it’s fine from the source, but you put it in a shot glass and suddenly it’s disgusting.
Huh. What about piss? Like if you pee in your own mouth directly, is that less disgusting?
Same reason you wear your socks the whole day but once you remove them they become eww. Human brain is weird in that regard. Probably has evolutionary reasons but from an entirely rational POV it’s weird.
It’s the quantity. And also, by the time you’ve filled a cup of spit, it has cooled and dried a bit, maybe even breaking itself down, resulting in an increased viscousity, giving it a very unpleasant mouthfeel.
Fluids that exit the body is like this. Only exception is blood from your own kin and tears. Anything else gets very dirty once out of body. Sweat, urine, vomit etc
Spit in a cup and drink it and watch how your peers react.