Declining a customer on the purely basis of sexuality is certainly odious, and that Lorie Smith is drawing attention to the fact she is declining the work on that basis does seem discriminatory. However, businesses decline customers all the time for a variety of reasons, some of which could be considered moral reasons. I knew a PR consultant who made a point of saying she’d never work with “immoral” tobacco companies.
Yet with a website, it’s very clear that the words are those of the client, not the designer. What the designer is doing is mechanical, not expressive. Should a motor mechanic be able to decline to service a car on the basis that there is a rainbow sticker on the back window? I think not.