in theory, it is not even known if a vaporeon is formed only by water because this was never said, what is known is that they control water.
if they were only made of water they would be completely transparent which is not the case as it is clearly possible to see the difference in color from the outside to the inside of the pokémon.
it sure would be pretty wet sex, but things like turning semen white would be impossible.
I think vaporeon is just a "simple" mammalian fish that controls water, I think this idea would only be possible if it was some power or something.