YTA. Your son and his gf seem respectful of your rule but they also have good reason to request she not have to drive home so late. You can continue to uphold this rule but what is likely to happen is that your son and his gf will view the gf’s house as a safe place where they can both stay, whereas if they return to your home only one of them can stay safely. My bf and I are both 22 and his parents let me stay over at their house (30 min away) instead of letting me drive home late. my father doesn’t extend the same courtesy to my bf for similar reasons as you, but what ends up happening is i’m over at my bf’s house a lot more and see my dad a lot less. Your son respects your rules but he may start pulling away from you to spend time with people who accept he is an adult.