Steven is the hero and this is his journey. It's a standard coming of age story. Hell, they even obsessively follow the "100% of the story is told from the hero's perspective" thing. Even when it's not about Steven (other gems getting into arguments because of what they did), it's about Steven learning that his guardians are flawed and that not everything is about him.
Greg is Steven's father. His life is kind of a mess, but he's a good dad and helps fill in a lot of the gaps that his nontraditional caretakers have. No, that's no innuendo.