Is it a sin to view naked girls? Not in a pornographic way, but as in like just girls without their clothes on.
Much depends on the purpose for which the viewing takes place. If it was a sin to see someone naked (boy or girl), then parents would have a hard time changing diapers and giving their small children baths. Doctors and nurses would have a difficulty time performing their duties if seeing someone without clothing was a sin. Though we try to avoid intruding on a person's privacy, when you live in a house with other people you might accidentally see them without clothing.
While talking about the need for every member of the church, Paul used the body as an illustration saying, "And those members of the body which we think to be less honorable, on these we bestow greater honor; and our unpresentable parts have greater modesty, but our presentable parts have no need. But God composed the body, having given greater honor to that part which lacks it, that there should be no schism in the body, but that the members should have the same care for one another" (I Corinthians 12: 23-25). Some parts of the body are not properly presentable in public. We treat them with greater honor by keeping them private through the covering of clothing. It is improper for a person to violate these rules of decorum.
Even when circumstances cause a person to see another unclothed, we ought to treat that person with the respect that we ourselves would like to have. "Therefore, whatever you want men to do to you, do also to them, for this is the Law and the Prophets" (Matthew 7:12). Averting the eyes, apologizing for the intrusion, offering something to cover themselves to restore their dignity, and the like are the ways we would like to be treated.
Let's put it this way, in the manner in which you think you could look at a girl, would you like it if someone looked at you in the same circumstances and for the same purpose? If not, then you ought not to be doing it.