if someone isn't willing or able to do their job because of their beliefs then surely they should just consider another job which is more suited to them?
They are perfectly willing and able to do their chosen job in their employed role, which is providing these services to female people.
There isn't a National Nail Bar And Massage Service in which all providers must be trained to a uniform standard providing the exact same service. An individual salon decides for themselves what services they choose to sell and who their customers are, and who they employ for those roles. When you're looking to spend your money you find a shop that will sell you what you want.
And yes, many workers providing services such as beauty treatments, massage, holistic therapy will only provide those services to females for a variety of good reasons.