Tell us, if one of the men had bought standard flowers, and mentioned to her that he was going to ask his partner to marry him with them, would she have been right to refuse to sell them?
No.
But if they asked the florist to specially prepare flowers meant to be used in a ceremony that was considered by the florist to be sacrilegious, yes.
See, asking someone to marry you isn't a sacrament.
Marriage is. And as such, being forced to design/provide/deliver materials to be used in a ceremony that is a vile sacrilege of one of the most holy rituals Christians participate in...is a violation of our right to religious freedom.