Why does Two-Bit tear up Marcia’s phone numbers?

After the movie, Marcia and Cherry make their way over to Two-Bit's place, hoping to get a ride. Their Soc boyfriends have gone off and left them, so they need someone to drive them home. On their way there, Cherry and Ponyboy strike up a conversation, where they discuss the differences between Socs and Greasers. Ponyboy feels comfortable in Cherry's presence, despite her being a Soc, so much so that he reveals a softer, more vulnerable side to his personality. He tells Cherry about the time when Soda worked in a stable, where he formed a close attachment to a horse by the name of Mickey Mouse. Ponyboy reveals how upset Soda was when the horse was sold and how he, Ponyboy, tried to buy it back for him.
The relative peace is shattered when Marcia and Cherry's Soc boyfriends turn up in their Mustang. Their presence is unwelcome to Two-Bit, who comes dangerously close to getting into a fight with them. After the girls speed off in the Mustang, Two-Bit makes his way to a poker game. One of the Soc girls, Marcia, has given him her phone number on a piece of paper, but Two-Bit tears it up. Two-Bit doesn't believe for one minute that a Soc would want to be seen with a Greaser. And besides, he thinks that the number's probably a fake anyway. It's just the kind of stunt that a Soc would pull on a Greaser.