I remember this story from sometime in the mid-eighties. A friend of mine claimed it happened in his biology class.
The day's lesson was on sex, etc., and the teacher mentioned how sperm is 80% sugar (or something like that) whereupon a girl asks out loud, "How come it tastes so salty then?" She turns beet red and runs from the room.
variation the instructor mentions that the amount of an average human male ejaculation is only about a teaspoon. One female student perks up and inquires, "Why does it make such a big mess, then?"
An Internet version began to circulate in 1997 in which a smartass professor answers the girl's question with, "Because the tastebuds for sweetness are on the tip of the tongue, and not the back of the throat."
Origins: This legend has been circulating by word of mouth, and more recently on the Internet, for many years. As is typical with urban legends, it is often related by students who claim to have been in a class where this incident took place, or to know someone who was.
This story most likely originated as a joke in a more chauvinistic era, intended as a quick, humorous put-down of women that combined their perceived unsuitability for traditionally male pursuits (such as science) with a trivializing view of them as mere sex objects. A similar legend, The Tell-Tale Swab, employs the same basic structure and theme.