Cosplayers are a visual representation of the characters, showing the fan interest of said characters and the cosplayer dedication. Especially those who put in real work in detail of their costumes. Couples take pride for their weddings or maternity or any other special occasions that is meaningful to them in cosplay. It varies from just being cute to having a meaning behind it.
By merchandise, that makes no sense that they don’t care. If a concept isn’t marketable or doesn’t make money, they wouldn’t give the licenses in the first place but given again Superman and Wonder Woman are marketed as a couple, it has to be known as such. The product whatever it is has the DC/WB label on it.
It goes from his/her pajama set, rings, to the bigger partnering with JPG for his/her fragrances and so on and so forth.