Though the sperm is a nonviable product, and the egg is much the same, the sperm-egg combination is both viable and genetically distinct from either of its parents.

Why is a lone sperm not viable, does it not thrash its tail with great purpose and competently carry its unpaired chromosomes?

If that doesn't persuade, why not rewind to the point where the successful sperm is just about to ploink through the outermost membrane of the giant ovum. Here we have a viable physical system that's genetically distinct from the mother. But that description also applies to her chocolate labrador. In fact, until the fertilised egg has anchored to the womb wall, her chocolate labrador is more viable (assuming she remembers to feed it). And every time a virile man passes a fertile young woman on the street there are trillions of genetically distinct and viable sperm-egg combinations within a 3-metre radius. Are they then morally obliged to go out for dinner that evening?

enforceability of such a law ... the loss of respect for all law that would follow if it were widely violated

Isn't the law to do with morality? Don't people want to be moral?

Would it be possible for sperm to be considered sacred while also acknowledging the fact that they typically do go to waste? Like, 99.999% (not sure how many nines to put there) of the time?

Not really. What if the Archbishop of Canterbury decided to dynamite all England's cathedrals except York Minster? Everyone in the Church of England would hate him, even the Bishop of Reading.

Sacred just means extremely valuable. It's used by some people because they're not allowed to value something unless it's loved by a supernatural being first. Something is either valuable to you or it isn't. I need water to survive. However, a particular raindrop would only be valuable to me if I was both about to die of thirst and I happened to pegged to the ground in exactly the right place at 3:17pm face up with my mouth open, etc.

There are some things that are supposedly sacred but in practice are not treated that way. For example, a scrawny cow wandering around a Delhi slum, chewing on cardboard.

If sperm are sacred they should all be frozen in sperm banks by law until some time in the future when we've constructed sperm paradise.

Saying, "It's not human because we reeeeeally need it," is probably not going to work

Has anybody actually said that?