Cherry and Marcia did not want to stay with Bob and his friends because the boys were drinking.
When Pony, Johnny, and Dally sneak into the movies, they are surprised to find two Soc girls sitting there alone. Their boyfriends are not with them, because they are avoiding them for the time being. The boys were drinking too much, and the girls did not want to be around them.
Cherry gets annoyed at the suggestion that the boys just got a little drunk.
Cherry looked mad. "A little? You call reeling and passing out in the streets 'a little'? Bob, I told you, I'm never going out with you while you're drinking, and I mean it. Too many things could happen while you're drunk. It's me or the booze." (Ch. 3)
This turns out to be very important. Since Bob was drinking and attacked Johnny and Pony in the park, he might have been more violent than he would have been sober. Either way, things got very out of hand. Bob tried to drown Pony, and Johnny attacked him with a knife and killed him.
Pony found out later that Bob seemed to have a drinking habit, and his parents let him run wild. Cherry stood up for Johnny and Pony with the authorities, even though Bob was her boyfriend.
They had interviewed Cherry Valance, and she said Bob had been drunk and that the boys had been looking for a fight when they took her home. Bob had told her he'd fix us for picking up his girl. His buddy Randy Adderson, who had helped lump us, also said it was their fault and that we'd only fought back in self-defense. (Ch. 7)
Cherry and Randy both imply that this never would have happened if Bob had not been such a heavy drinker.