A Soc and a Greaser weren't supposed to get along. They were too opposite things, too different to be friends.
But Randy was as close to a Greaser you could be while still being a Soc, and vice versa with Ponyboy. The only things that marked them as Greaser and Soc were their clothes, hangouts, and music preferences.
They were more than friends. Didn't let anyone know though, because their group of friends already hated them for being friends with the enemy.
So imagine what would happen when they told their friends that they were dating each other!