It doesn't matter whether you support same-sex weddings or not - opposing it and not wanting to be involved in one has nothing to do with discrimination based on sexual orientation. 
 
To those that say, "he advertise hosting weddings and should accept all" the obvious answer is that his definition of wedding is not the one of the State. How do you call the procreation-oriented union of a man and a woman, before it was redefined? There, he is offering to host these, and of course does not care about the sexuality of the people involved. 
 
He is anti-gay "marriage", not anti-gay.