The only situation where I can see gender preference being relevant are jobs that exploit (mostly) women. When heterosexual men go to a strip club, they don't want to see a dude on stage. Likewise, when heterosexual women go to see the Chippendale dancers, they don't want to see a woman or women on stage. Then there's restaurants like Hooters and The Tilted Kilt that exploit young women's looks to attract male customers. I'm not sure if there are similar restaurants exploiting young men's looks, but I wouldn't be surprised.
I guess if you think those sort of businesses should be allowed to exist, then employers hiring people to work in such places should be allowed to add a gender preference when looking for employees, because gender (and appearance) clearly matter in the performance of those jobs. Come to think of it, if you're casting for a movie/play/t.v. show, you would hire according to gender too. George Clooney wouldn't make for a very convincing Amelia Earhart for example. But in jobs where a person of either gender is not part of what qualifies one to do the job, I would say no, employers should not be allowed by law to add a gender preference when searching for an employee.