My problem there is that the topper is usually plastic. It's not something he makes, it's something he stocks. He'd mostly be within his rights to decline to stock all the styles, he can claim he doesn't have the demand to keep those in stock. Now if he's making them out of fondant or whatever, then serve one, serve all. Also, if he would normally special order items to place on any other wedding cake, he would then be obligated to special order for a same sex couple. So if they then asked him to special order their topper, he would need to. But if it's not his practice to special order items from outside for customers, then he could decline to do so for a gay couple. That wouldn't seem to be the brightest way to run that sort of business, though.