You may want to introduce a fourth individual in your example, so it's not limited to species (mostly amphibians and fish) that are able to morph sex or hermaph

You may want to introduce a fourth individual in your example, so it's not limited to species (mostly amphibians and fish) that are able to morph sex or hermaphroditic species (some molluscs, snails, etc.). Otherwise, the example is trivially true for all mammalian species that I'm aware of; for humans, if Arthur and Betty can easily breed fertile offspring, and Betty and Clive can easily breed fertile offspring... then Arthur and Clive are going to have a very low breeding success rate without technological assistance.