While I am not a breeder and do not have a vast experience base I want to put in my 2 cents worth.
Genes have the ability to adapt. Species of the same genus may be able to breed in a confined situation, and do.
Most call this hybrid. Some can breed further, others are sterile.
A lot are between cultivated line bred varieties so the difference are purely Visual and aesthetic.
Some people like this ability, others shun it.
But in the wild is this not the way species evolve?
Another question I rarely see put forward in this discussion is:
What is the breeding history of this fish I have in my tank???
At one point it came about from a crossing of two other different types of species of varieties of the same species.
It may have been in the wild (whether it is accepted as not common) or was in an aquarium before your time.
To not acknowledge this seems to lead to a one sided argument where the few line breeders on one side, and a few less knowledgeable/less experienced individuals selling on mixed line offspring.
Dogs are mixed all the time and no one culls the mixed crosses and causes a big stir.
The pure bred breeders still sell their wonderful creatures and the ones who don't care keep mixed breeds.
It is all swings and roundabouts.