ok my point of view as guy.
you went wrong by taking that drink ...

if your with someone else do not take drinks from someone else.
guys buy women drinks so they can talk to them while they drink it. nothing else is expected from the drink except a talk, which THEN the talk can lead to something else.

If my girlfriend took a drink from someone else while i was with her ... id be very upset with her, I find that very wrong

I dont think the guy did anything wrong.