Because the right to "discriminate" is a constitutional and God-given right.
I would not hire someone who drank too much on weekends or was a serial adulterer or who spent their weekends whoring in Sparks Nevada, much less someone who advocated for that lifestyle.
Does your brand of conservatism allow for those people to sue me for "job discrimination" because I don't approve of their lifestyle?
As a private business you should be able to hire whomever you like.
Now you tell me, is a church conservative if they let sinners (gays) into the service?