My boyfriend (30M) looked at another woman and I (26F) feel insecure

My boyfriend, his HR director, an employee, and I went out for lunch. My boyfriend and I were seated facing into the restaurant, while the other two were facing toward the back.

There were two girls sitting diagonally opposite us. I noticed them when we sat down but didn’t pay much attention. My boyfriend, however, did.

They got up and started taking pictures. For context, they were wearing short, open-back dresses, and my boyfriend kept looking at them. I think he turned his head around 15–20 times over about 30 seconds to look at them while they were standing up taking pictures around the restaurant, and even when they sat back down.

At one point, while they were at their table, one of them bent down and her backside was clearly in my boyfriend’s view, and he was staring at her. Shortly after, she went to the bathroom and made eye contact with me as she walked past.

I tried to keep it in, but I ended up asking him at the restaurant why he kept looking at them. He seemed shocked that I noticed and didn’t really give me an answer, but I guess I already knew.

I’ve already been feeling quite insecure, and I think this made it worse.

I keep bringing it up because whenever I think about it, it makes me feel awful, and we end up arguing. He says it was just that one time and only happened once.

How can I process what happened and talk to him about it productively?