Outside Crushes During a Relationship

So I'm currently in a serious relationship of about a year and a half and going. I also happen to have a little bit of a crush on a coworker. Like don't get me wrong I love my boyfriend and there's no contest between the two; my boyfriend wins out every time. I just feel like that shouldn't be a thing that's happening to begin with. I told my boyfriend about it and he's not upset…like at all. And I feel like he should be, like even though he's my number one choice shouldn't I not be like attracted to other people? I just feel like I betrayed him somehow and I dunno if he should be upset or if I'm overreacting