Field work on some fishes has discovered internal parasites (this sounds like Alien) which had damaged internal organs, including the reproductive system. In one case in Venezuela, at the end of the rainy season, the fish in question no longer had reproductive organs at all!

The fish could be very full bodied, but that is because of the unwanted guests filling up the body cavity. Obviously those fish are useless as breeders, though if they survive the "cure" they might look nice in a community tank.

I wonder about advanced cases of Camallanus. Young guppies, preventatively treated when their parents are discovered to be infected, should be ok if treated soon enough. Probably would have to isolate or at least observe a parent.

Previously mentioned on guppylog are cases where moved livebearer females dropped prematurely. Some severe traumatic situations were so damaging that those females were unable to have fry again. :(

I'm quite sure there are maladies which can sterilize guppies if the damage is extensive enough.

A companion question to yours: Are there medications which can sterilize fish with regular dosage or over-dosage?

I suppose that makes a strong case for following dosage instructions, actually figuring out how many gallons the tank really holds, staying with treatments for the prescribed time and especially removing the medication after the medicating period is over.