Grease is really a "Gift of the Magi"-style story
Too many people focus on Sandy becoming a whore to get her man. She allegedly gives up being a good girl in order to win the bad boy of her dreams. This is a girl who is explicitly identified with Sandra Dee, the innocent Ingenue star of the 1950s—even more so in the play, where her last name is "Dumbrowski" (making her literally "Sandra D"). Blonde, sweet and even a cheerleader—how great a fall for her to go "bad" just for a boy.
What people tend to forget is that Danny undergoes his own transformation. Realizing that he can't have the "good girl" as he is, he reinvents himself to the point of becoming a letterman in track and field. Things have changed some in the decades since the days in which Grease is set, but back then, being a letterman was a big deal—it didn't matter the sport, you were the Big Man on Campus, you were college material, you were the definition of the guy parents wanted their daughters to date.
So consider this—Sandy becomes a "bad girl" to get her bad boy, and at the same time he becomes a "good boy" to win his good girl. She hasn't "fallen to his level"—they've exchanged places. But unlike the usual "Gift of the Magi" Plot, neither has actually given up anything in the process. Instead, they've grown, expanding themselves rather than replacing themselves. They each now partake of both sides, and can choose which to embrace when. Danny may have peeled off and discarded his letter sweater to sing "You're The One That I Want" with Sandy, but nothing says that's permanent—he's just matching styles with the girl of his dreams. And it'll go the other way, too—they're each now comfortable with the other's world.