They don't belong together. They both know that. They know he belongs with a cheerleader, and she belongs with some choir geek. He belongs with the girl he impregnated; she belongs with the guy she's been pining for all along.
They're so different in almost every way. Their religion being one of the only things one could say is similar between them.
But when they get together, and his hands tangle in her hair, their tongues battle, and her legs wrap around his waist, they're the same. They're the same in every conceivable way.
It's his smile and his eyes with five colors in them that drive her crazy. It's her voice and her ability to not give a damn what anyone else thinks, because she knows she's going to be star that drives him crazy.
So even though he should be with Quinn, and she should be with Finn, Puck and Rachel defy the odds and become one.