First of all, when Penny tells Leonard what Zack’s been up to lately, he’s gets jealous immediately.
Leonard then tells him that he doesn’t want to seem immature and petty, so he’s agreed to go to the dinner.
Sheldon clearly isn’t convinced Leonard’s reasons are valid, but he’s grown lately so he doesn’t say anything.
She manages to stop him from thinking about it too much in the moment by distracting him with sex, but the next day, when he’s driving Sheldon around, Leonard tells him why Zack’s new life bothers him so much.