when someone would get a "pie", they would get anything from you sex wise. this can be used in comparison to someone not as worth for "pie" as to be a tester. testers like the "try before you buy" in Tesco's. So if someone is a try before you buy, you're not commiting to a full pie but simply testing. A full pie is where you are definately sure they are worthy of your whole body.
Emily said to hannah, "hes bum, he can have a slice of my pie" hannah replied with "i'd prefer to try before you buy, then buy the whole pie". The boy looked at them confuzed.