Is it normal to “check out” other people when you’re in a relationship?

I noticed that my boyfriend follows beautiful girls with his eyes when we’re together. I know what his type is, and I also know when a girl is attractive. I know who and what will catch his attention. I noticed his pattern. He sees a beautiful girl, stares at her for 2-3 seconds, turns away, then looks at her again when she walks past us. We were sitting at a table eating dinner, a girl walked past us and I knew that he would check her out. He stared at her face, looked down, then turned his head a bit when she walked away. Yesterday we were about to get on the bus and I noticed a girl getting off, and knew that he would stare at her. I was standing in front of him, and not surprisingly he had his head turned to look at her as she walked away. It makes me sad to think about. I know that I also acknowledge attractive people, I may look at them for a few seconds but that’s it. I don’t turn my head to follow them as they leave. I don’t want to make my boyfriend feel uncomfortable or unappreciated. I don’t know if it’s normal, I don’t know what to do if it’s not. I’m not sure if what i’m doing is normal as well.