We are a "visual" species, and are also blessed with a 24/7 sex drive. So it's not at all unusual for you to be imagining the view of other bodies naked. The actual view of another person naked is often a signal for sexual activity in our species -- at least, we have made it that way by hiding our bodies until we are about to be sexual.

Sexual thoughts occur frequently throughout the day in both males and females. The drive to reproduce is the strongest and most primal drive we have, and so we are supposed to enjoy contemplating how the sight of others of our species makes us feel -- we are supposed to like it!

The problem would be if you begin to act on those fantasies. If you were to walk up to a boy or girl and wordlessly begin to unbutton their clothes to get a better look, that might be a problem! Perhaps keep it to yourself until you are able to use that desire in your sexual life. But imagining to yourself is not a terrible thing to do. It keeps the erotic and sexual pathways active.