"Not tonight" suggest openness to the idea in the future. Think about it would you say "not tonight" to something you were dead set against? "excuse me, would you like to drink some sewer water"? "oh, not tonight" (said no one ever)
It also can be interpreted as "Tonight I want to have a drink only with you, because I want to be more intimate, only between us. We might share friendly drinks with others some other time."
Why is your interpretation more accurate than mine?
