There have been some good comments in this thread, including many based on anthropology. I've studied a good bit of that, but I think the answer lies at a much more basic level.
Were I to try to spread those sperms around, as might be optimum, my wife would kill me--hence, there is no survival value to that strategy no matter what the anthropologists say!
Religious belief does not constitute scientific evidence, nor does it convey scientific knowledge.