What is "natural"? Is keeping any animal in captivity "natural"?
Fancy fish - different than what is found in nature - have been the focus of captive fishkeepers since, say, 200AD when people started to develop colored carp (koi).
Fancy fish - both hybrid and line bred - make up a big portion of fish kept in aquaria: fancy goldfish, fancy bettas, flowerhorns and parrots, fancy angels, long-finned fish, fancy livebearers, etc. Is keeping a long-finned betta wrong? It would get snatched up and eaten in a minute in its natural habitat...but it sure looks cool!
People get personal taste and ethics mixed up when discussing this topic. Just because one person likes classical music and another likes rock doesn't make rock music unethical...
Matt
PS The argument that hybrid fish are bad because irresponsible / unscrupulous people pass of hybrids as "pure" fish has nothing to do with hybrid fish and everything to do with irresponsible / unscrupulous people. Do you think that a person who's willing to sell small flowerhorns as something that they're not would hesitate for a moment to sell tank raised "pure" cichlids as wild? Or small cichlids of a common species as a rarer one?