Is it rude to talk to someone else when with your significant other?

Last night my boyfriend of 2.5 years and I went out to a bar/grille for dinner. We'd never been there before. There was another woman at the bar sitting next to my boyfriend. He made conversation with her right away, and it turned out they had a lot in common. They liked the same sports teams and they actually went to the same high school--she was a year older than him. They weren't flirting or anything, but he basically talked to her the whole time, and I didn't feel included at all. I tried to include myself from across him but it was like I was interrupting every time. He didn't put his arm around me or his hand on my leg or anything and talked to her from the time we got there until the time we left. I felt disrespected by this, because if you're at dinner with your girlfriend the majority of your attention should be on your girlfriend, not on anyone else, especially another woman. Talking is one thing, esp if all three of us are all included. But to spend the whole night talking to someone else to me is disrespectful because I was left out. Am I just being jealous and unreasonable?