Think about this logically people: WHAT in semen are you expecting to whiten your teeth? The salt? Nope. Harmful for teeth, actually. Same for the sugar in it.
That's about all there is to sperm, aside from water and protein from the sperm itself. I don't think I've ever heard of a whitening toothpaste containing salt, sugar, or water as the main whitening ingredient. So before you get defensive, get smart.
Semen does not whiten teeth. In fact, you should probably brush right after, or risk tooth decay.