"Exact equivalence"? BULLSHIT.
Obviously, a Jewish food vendor who keeps Kosher is not going to have pork anything on hand; a gay couple wanting to wed is likely to want much the same kind of flowers or cake that a straight couple would - with the possible exception of the little figures on the top of the cake.
For that matter, in some small towns there might BE only one nearby baker who does fancy wedding cakes - and there have been several cases of this kind of anti-gay bigotry
in the news where a gay couple had dealt cordially with the vendor, not had ANY idea there would be a problem until the subject of their wedding came into it - so your straw man of "When a gay couple TARGETS a Christian business...." face-plants right there.
It is long held and settled human rights law in this country that when one opens a business to the public, one must serve ALL
of the public. Period. That means all the skin colors in the human spectrum, atheists, Muslims, Hindus, Buddhists, Sikhs, Jews, Catholics, Protestants and so on... oh, yeah - and LGBT people too.