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.
You are swallowing that saliva during the day. Usually we make 0.5 and 1.5 liters daily (for us measurements 2 to 4.2 cups). And that needs to go somewhere. And it is very important part of our oral hygiene too.
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.
I had some oral mucositis that caused some excessive saliva production. There was one time I accidentally swallowed a mouthful of it, it immediately caused me to vomit.
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.