In my opinion being sexy (=what makes me horny, what can give me a hard-on) is definitely not the same as being beautiful. Of course, sometimes sexy and beautiful happens to come in the same package, but in my opinion there's no strong necessary connection.
Sexy is a matter of behavior and attitude: like knowing how to shake your ass, or, more generally, how to move your own body, and showing that your're open to sexy business.
Beauty in our European tradition has very strong connections to static proportions. An example: Michelangelo Buonarotti's monumental sculpture "David" is beautiful because of his perfect proportions, but is he SEXY? He most certainly doesn't give me a hard-on.
He can't shake his ass...