Sharpsburg is considered a draw. Lee withdrew into Virginia, McClellan failed to destroy his Army.
Antietam spelled the end to Lee's first campaign in the North. He was forced to retreat. And in doing so the south lost any chance of European recognition. Antietam spelled the end of any slight chance the south ever had of winning their rebellion. It was a major defeat.
Change "McClellan" to "Meade" and you've just described Gettysburg. Was that a draw, too?