It's one of those stupid things we do as humans.
"Dont touch that, it's hot!," your parents say, the kid then invariably sticks their finger in or on whatever they were told not to.
"Dont look! It's awful." We all look.
I wonder if your girlfriend exp5lained why she didn't want you to speak to this girl. What she was afraid of.

And golden I suggest if this was a girl saying her boyfriend had told her not to speak to someone, the feminazi's would be out in force saying, " he's not the boss of you, you can speak to who u like, it's not like your doing anything."
Also I can pretty much guarantee they would assume he was hiding something or up to something shady.

So if it's a man its controlling, but if it's a woman saying it it's intuition? Really?

OP, You did something stupid, on the grand scale of offenses, talking to someone your girlfriend doesn't like isn't massive. Her reactions are interesting though.
As she obviously didn't have faith in you in the first place, it still all seems a little OTT.
You fessed up in the end, again better you had told her right away.

Thing is you ask someone to do something like that without explanation, they are gonna want to know why.

I certainly wouldn't be told who I can and can't speak to, and I wouldn't dream of telling my fella who he can't talk to.
To me she sounds a little immature and in need of addressing her relationship expectations.

Yes he was stupid, and should have dealt with it differently.
Next time don't make a promise you can't keep OP.
IMO if it bothers her that much she should walk now not drag out the punishment needlessly.