An Argument For Ethical Skepticism

I'm teaching an introductory ethics course this summer and we're reading Russ Shafer-Landau's The Fundamentals of Ethics. In the introduction, he considers ethical skepticism and mentions that some people think that ethics is just in "the eye of the beholder" or that it's "a bunch of make believe."  Here's one argument he considers for embracing this kind of skepticism about ethics:

(1) If there were a universal ethic, then that would make it okay for some people to impose their views on others.
(2) But that's not okay at all.
(3) Therefore, there is no universal ethic.  

What do you think about this argument?  Are the premises true?  How might someone respond to this argument and what do you think about such a reply?