When does the right of an employer to choose the person he feels is best suited to his business needs become discrimination?

When does the right of an employer to choose the person he feels is best suited to his business needs become discrimination? Every employer or recruiter knows the type of person they are looking to recruit, so is that discrimination or simply an employer knowing what he needs for his business, which he operates with his own money. There was a café in Albany, WA last week, criticised for advertising for customer service person, but wanted female only. His reasoning being that he felt females were able to able offer better face to face customer service. His mistake was stating so in the job ad. What would have happened if he had left that part off, interviewed everyone, but them simply employed a female as that is what he wanted? Would it still have been viewed as discrimination, as he interviewed male and female? So, if employers are still easily able to employ the person they feel fits their business needs, then is this discrimination?  You can hear mu discussion on TripleM radio regarding this topic.