This would seem to be nature's way of saying:

"No soup for you!"  (sorry Ciara.)

On the other hand, I have had women tell me that the semen from some men makes them feel euphoric after sex.  

That would seem to indicate: "This is the one."

Any chemestry majors out there?

We need more chemestry majors here.

Thruster, I'm sure you meant reasons you were GIVEN by women for not swallowing.  Just wanted to lend a helping hand, so to speak