Why is this? In the Pokémon universe, you are either a human or a Pokémon. There are no animals in the Pokémon world. Thus, we can infer that humans are the odd species out in a world full of completely different creatures. Now, if humans, being a unique species, can be homosexual, what reason would could there possibly be to assume that every other gendered species on Earth can be homosexual?
Now, I'm no Pokégeneticist, but I believe that heterosexuality is a basic aspect of a Pokémon's genes. It would be extremely unlikely for any Pokémon to not have it, thus, Pokémon homosexuality is a genetic abnormality. Genetic abnormalities are not represented in-game, so why would homosexuality be?