finding someone else attractive, feeling guilty☹️

Okay so I'm starting to feel a little bad about finding a buy in my biology class attractive. I've been with my boyfriend for 2 and a half years and hes the only guy I've ever been with (we're both 19). I don't ever plan on cheating on him nor do I think about it but I just feel guilty for this. And this is weird because I've found guys attractive with no problem, or guilt, this is the first time I feel like this. I don't know what to tell myself. I need some womens advice.