You met the boy, dated the boy, you’ve met each others people, and you have a few things at his place. Slowly but surely the relationship is progressing until one day he presents you with a small box. Is this what you think it is? It is shiny and it is metal but alas it’s only a key to his place. Why do I say only when it should be a significant step in a relationship? Because it’s mired in ambiguity and opportunities to screw up.
Obviously he’s thinking of this as a step forward but have a conversation with him about what a key means. Is it for emergency access? Is it so that you can lock up on Sunday morning while he goes for a run? Is he expecting you to be in lingerie with a martini in your hand when he comes home from work? Have him be explicit about what is and isn’t acceptable so there are no miscommunications (and no hurt feelings when you try to do something nice and he freaks out about you “trying to take over” his life).
Once you’re given the boundaries – respect them. Do not use the key for snooping (people who snoop deserve what they find) or any sort of homegirl investigation (you’re not a private dick). Do not use his place to hang out, store stuff, drink his booze, or entertain friends. Think twice before using it for anything he doesn’t already know about and approve of. A key is nothing more than temptation in your pocket. Consider it before accepting.