
Originally Posted by
ipso facto
It's not a matter of whether 'the sperm' is healthy. You have at least seven semi-different varieties of sperm in the typical male ejaculate. Some are there to break the membrane of the ovum; some to send of pheromone homing signals; some to block the tubes from sperm from other males; some to impregnate; and so forth. They are the same sperm with different proclivities, and if their metabolism is altered, it alters the whole dynamics of the impregnation process. If the membrane breaking sperm are weakened, then your genetic carriers never his their mark. If the blockers are weakened, then other males may supplant your sperm (you hope this isn't a problem if you are married). And so forth.
The effects of drugs are not just on quantity and quality of sperm -- they alter an entire ecosystem necessary for impregnation.