You're seriously asking this? It's because they're fictional characters that don't have minds of their own, the creators make them do what they want. If a character is homosexual, it's because the creators made them homosexual. Now, let me remind you, everything in a story is supposed to serve a purpose. That includes everything about the characters. Making characters gay does nothing for the story. It's like Dumbledore from Harry Potter being gay, that was completely random, and served no purpose. Same thing with Korrasami, it served purpose other than to give millions of guys boners and make girls scream, "Yeah! Lesbo girl power for the win!"