rationalskepticism.org

Actually there are two completing forces at work here. Semen is composed of sperm, water and several proteins and enzymes. One protein is a coagulating protein, the purpose of which is to make sure that the semen stays in a mass inside the mate and doesn't run back out. One of the enzymes is a de-coagulating enzyme which is supposed to dissolve the coagulating protein after some time, releasing the sperm bound up in the coagulated mass. This combination dramatically improves the chances of impregnation. In the shower the de-coagulating enzyme gets washed away rather quickly, leaving behind the coagulating protein, thus instant spermy glue. As some have noted, the temperature is not critical. However, how many times you've tried this in a day does matter. It takes time to produce the coagulating protein (and everything else but apparently especially the protein). After the first one or two tries, your semen is going to appear much more watery (very little protein) and have a lot less cloudy whiteness (much less sperm {whiteness}). And of course, the younger you are (assuming you're past puberty) the faster you recover your stickyness.