The same could be said of a zygote, which is actually less animate in its behavior. The problem with trying to draw the line at any particular point in the process is that it's arbitrary. Even scientists who specialize in this sort of thing admit that no strict definition of "conception" is possible. Humans arise as the result of a myriad of causes and conditions.
Incidentally, sperm cells (or semen generally, as they couldn't see the individual cells) in antiquity were thought to contain the entirety of the genetic material for life, which was merely hosted by the female without her contributing anything. Hence ancient prohibitions against spilling seed, as well as ideas about hereditary descent that would seem bizarre to us today. If we still held the ancient view, which Church doctrine used to accept as correct, then male masturbation would be tantamount to abortion, and nocturnal emission to miscarriage.
It's silly, yes, but it goes to show how arbitrary these ideas are. There's no objective scale for determining personhood. Me, I'm tempted to put it at the point where someone learns to speak in complete sentences.