if I recall, ejaculate has a measure of chlorine in it. I recall waking up to wet-dreams and smelling bleach (a smell I HATE to this day)--and thinking it was in the laundry. years later I learned it is a chemical in ejaculate which is (I assume) disinfect the area so the sperm don't die off. Now, think about what bleach does un-attended/un-rinsed on anything... fades clothing, ruins finish on furniture, and yellows your white t-shirts.
Best you can do, is rinse it as soon as you can to keep it from chemically reacting with whatever it touches.
From the interwebs: Semen normally smells like ammonia, bleach, or chlorine. Semen is about 1 percent sperm and 99 percent other compounds, enzymes, proteins, and minerals. Many of these substances are alkaline. This means that they're above a 7 on the pH scale, which is measured from 0 (highly acidic) to 14 (highly alkaline)