There’s nothing wrong with that person unless they are causing havoc on purpose. Everybody who calls that person sexy is crazy.
No, it is not OK to call someone else sexy. I avoid calling Pierce Brosnan or Hayden Christensen sexy even though you and I both know the fact of the matter.
I’m a sati savitri mashriqi aurat who has no right to admire another gentleman for the risk of sounding obnoxious, but it is perfectly in order for men to oggle at Angelina Jolie. I understand. 
But no, the use of that ess word is strictly forbidden. 
In fact your thread is going to burnnnn.