The buyer’s desire for performance isn’t what requires the florist to act. The florist offers her services when she opens the business. That is what requires her to act.
An interracial couple has the same choice in having an interracial wedding. Do you favor allowing discrimination against interracial couples as well? Interfaith couples? Or couples of a faith different from the florist? You need to support discrimination in these cases if you favor it here, in order to be consistent in your ideology.