Both of them shouldn't be with him, because both of them need a break from dating. What happened to Quinn getting pregnant and deciding that she shouldn't just focus on chasing after guys or popularity? What happened to Rachel in that sleepover scene with Mercedes and Kurt, deciding that being single so she could focus on her career wasn't such a bad thing? Plus, knowing Finn, I don't think he's quite ready for a girlfriend with as much, um, instability as Quinn or Rachel.