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.