I've never really understood this. Some restaurants, or clothing stores, have wait staff that are all women. In a massage place you can pick the gender of your masseuse - so surely a massage place that was already well staffed with one gender would be justified in gender discrimination. If I'm hiring a baby sitter for my daughter, I'm not going to hire a male person, and I'm sure there are many similar examples. I get that many times there is no justification for hiring preferences based on gender, but surely sometimes there is.