I also have a mixed mbuna tank and I am sure there is plenty of mixed breeding going on. While these fish will never leave my house, I don't cull the fry that survive. While I agree that these "mutt" hybrids should not be sold and I would not even give them away, I did purchase my Parrots and have grown to like how they look. My wife loves them and that is really enought for me to like them.
I think there is some sort of line between everything goes (like fish mutulation and dying and even tattoing to nothing goes (like PETA). I read PETA stand on fish keeping and some parts made sense. While I did not agree with all of it, I can see their point.
These fish are not in nature, we have chose to keep them in our homes in a prison made of glass for our enjoyment. While we should not harm them, if in captivity they breed differently than in the wild, what is the harm. As long as these fish are not mixed into the rest of the fish trade, then I really see no harm. I guess people could just not mix certain breeds or try to keep all males (or all females), but I think accidents will still happen. Killing off the fry from these "unatural" pairs is worse than letting them live.
Too me there are larger concerns that face this hobby. Things like the sale of large fish to people with small tanks, the mixing of fish that will attempt to kill each other, the transportation of fish before and after the sale and poorly maintained fish tanks. I also think their should be a ban on those freaky clams that open up with an air bubble (lol).
Fish are often hybridized across species. Think of it like horses and donkeys making mules. It's not like different breeds of dogs, cats, or horses. That occurs with fish as well (like different types of bettas for example) but you have to recognize that when you mix two species you are hashing together mismatched genetic code. Mules are sterile, for example.
I don't think its horrible to allow fish to hybridize. But I think the offspring should be treated like an oddity for a collection, not evolution. I also don't think that making two creatures mate to produce unhealthy offspring is ethical. But that stands for breeds of animals as well. Some dogs are unable to reproduce through copulation because they are so disproportionate that the males chests would crush the females pelvis, and have to artificially inseminated. Some breeds can't give natural birth because they can't crown a single puppy. That's a sign that those dogs shouldn't exist, to me. But I digress.