Perhaps a male perspective would help.
Relationships like yours, one that is steady and committed, typically have very clear rules and expectations about acceptable behaviors. Like two hands tightly clasped together, two people who are bonded with each other should not allow anything between them. To allow other relationships to take precedence over your own is not acceptable in a committed relationship.
Let's look at this one issue at a time.
First of all you were concerned about his behavior. This feeling should immediately grasp the attention of your boyfriend. Just by you saying you are uncomfortable should not have resulted in anger on his part, but with concern and introspection on his part. (Why did I ask her out for a drink? Was I thinking of my current girlfriend's feeling when I did so?) And, if he knows you well, why would he feel that drinks with another woman were an option? There is trust and openness, and then there are poor choices.
Most successful couples that I know and work with establish very clear boundaries about what is appropriate and what is not. Drinks, alone with a member of the opposite sex...that typically is not acceptable and even if innocent disregards XXXXX XXXXX and possibly sends the wrong message to this other woman.
I see nothing wrong with standing your ground on this. This is not about trust. It is about what is acceptable in a real relationship. If he is the type of man who puts you first, (and he sounds like he has been) he should be concerned about you enough to stop this behavior immediately and to set boundaries that he will not cross.
One such lapse in judgment is certainly excusable with a promise that it will not occur again. But this is not just jealousy on your part in my opinion. It is a need to be clear with expectations and to set clear boundaries on both of your actions with others. How does this occur? Simply, things need to be spelled out and expectations frankly discussed. There is no absolutely best way to do so, and the stage of your relationship, called committed reciprocity, requires that you both lay all on the table. Yes, conversations like this can be difficult. But as long as they are planned. (over dinner is good) and timed (no more than 20 minutes each) and they are done in compassion and concern, they work out well.
I think this will work out. It is a pretty common issue for males who need to see that not all that is "innocent" is acceptable. Steven