It would be unhealthy for him as an individual if he did NOT have other interests. It is healthy to find attractive people attractive. You're not the only female on the face of the planet, and there is no such thing as the "most beautiful" person on the planet because beauty is subjective.
To expect your boyfriend to not look at other attractive women would be selfish and unrealistic. His libido isn't going to magically shut off just because he is in a relationship, that isn't going to change. But what can change is you can afford him some more privacy.
If what he does is not affecting your relationship, then by definition it isn't harmful, but if you go snooping around through his personals, then you'd best be prepared to find personal things. He's a normal young adult with a healthy sex drive, but he's not cheating on you; that means he's a healthy and loyal young man.
There is nothing wrong with acknowledging the attractiveness of other human beings even if you're in a relationship. It is psychologically unhealthy to lie to yourself that someone else is attractive because you think that if you admit they look good that it'll tempt you to cheat; that's just flawed thinking, and it is a sign of a lack of trust.
Trust is one of those things that either is or is not; if you go through his things, you don't trust him, and if you trust him, then you'd feel no need to go through his things.
But to answer your question directly; yes, it is perfectly normal for a healthy person to want to look at other healthy people.