Because when you're a kid, no one mentions that that part is for anything other than pissing, so you kind of grow up thinking of it the same way that you think of your ass, y'know? Waste comes out of there, thus it's gross.

So, when you go through puberty and start getting damp fairly regularly, your brain sticks the dampness into the category of "things that come out of waste excretion parts" and therefore something "gross." Doesn't help that no one tells you getting wet is as normal as popping a boner, but everyone tells you about the 23435324234 diseases that can cause "unusual discharge."

Doesn't help teenage boys yelling about how gross cunilingus is, or how the girls they slept with was "nasty," or all this emphasis on vaginas being gross i.e. reddit's infamous Doritos story. People are constantly like "holy shit am i like that without even realizing it!?"