so what... they're not refusing to serve homosexuals they're refusing to create something which they're opposed to.... if we go down that line then there are plenty of other silly examples such as the Muslim baker Jesus cake one which ostensibly should be covered by the same law.
A heterosexual could order a wedding cake, a homosexual could not order a wedding cake. That is fundamentally discriminating due to sexuality. Much like I couldn't run a guest house and only let black guests book the attic room.