I have heard of it, but I wouldn't know. It might be possible with all the talks about how enriched in calcium it is, so it might contribute to bone/teeth strength? ::shrugs:: Why do you want to know anyway? Thinking about changing your toothpaste brand?
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.