Throwing out milk.

From a microbiological standpoint, you're doing the right thing. The reason food and drink have expiration dates is because you're not supposed to consume them after that time. No one should guilt trip you for following safety protocol; however, if you're filling a 12 oz. glass, drinking 10 oz., leaving the other 2 sitting on the table for a couple hours, then throwing out the rest when you realize you forgot about it....that's another story. Just try to not let something like that happen frequently and, if you can't help it, take less and refill as needed.