AITA for being hit on by a stranger and not immediately saying I had a bf?

I (22F) was hit on by a stranger who made some small talk and asked for my number, I said no and he immediately walked away. My boyfriend (22M) of nearly 3 years is upset since I didn’t say “I have a boyfriend” and he claims that this should have been the first thing I said in response to anyone flirting with me. My reasoning is I wasn’t sure if the guy was hitting on me and didn’t want to be presumptuous. When I said no, he left so I didn’t give it another thought. My boyfriend says I betrayed his trust by talking with this guy and only saying no, I think he’s overreacting. AITA?