Most of the porn tubes with a search facility will features these but, as has been said, they go back to the 70s and 80s. In some cases not particularly convincing - the penis is erect from the word go and doesn't always seem to be stuck in the right place. On coming, a stream of thin white liquid that looks like watery milk sprays out in what I would call excess amounts. At least that's what I noticed in the clips I've seen.
My two bobs worth.