Why did this weirdo baker decide to look up his customer's Facebook page to begin with?
Yeah, that is weird as hell. I hope the backlash puts his business out of commission.
Anywho, I have a pretty clear-cut position on this. If you don't believe in gay marriage and don't want to "aide and abed" a gay marriage ceremony, like a wedding, you have the right to refuse service. However, a birthday cake is not religious in nature. The private life of the two women involved is none of his business (it would be if he had to bake a cake involving their private life, ie, a wedding) so he lacks the right to freedom of association in this case.
If he can deny service to these two women, why not deny service to a pregnant teenager? Or a recovering alcoholic? The public means the public. A gay wedding is not the public, but a gay couple entering the bakery for a simple birthday cake is.
Both are private events (a birthday celebration is hardly a public affair after all), so if a baker if to be compelled to provide a cake for a birthday (and in this case, the birthday of a gay woman's spouse, which is surely an affirmation of their love and their relationship), then it isn't really much of a stretch to compel the baker to bake a wedding cake. Given that Christian opposition to same-sex marriage stems, at least in part, from the view that homosexuality is basically a sin, and that homosexuality is a false form of love and cannot provide the foundation for any kind of family life, then surely a woman buying a cake for her spouse (as I said before, an affirmation of said 'false' love and family life) falls into the exact same category as buying one on behalf of herself and her spouse to be.