To me it boils down to simply this: Wonder Woman is supposed to be an iconic superhero in her own right independent from the others, part of the Trinity of the other two iconic superheroes in Superman and Batman. And while admittedly her franchise is not nearly as big as the other two, she does have her own mythology and supporting characters. By having her be in a romance with Superman, you've essentially turned Wonder Woman, who again is supposed to be an icon superhero in her own right, into a supporting character for Superman. She becomes part of Superman's story instead of her own story. It's essentially DC all but admitting that Wonder Woman is incapable of making it on her own steam. And for those who say "Well, why wouldn't it be the other way around?" the answer to that is "Take a look at which of the two has more comics, movies, tv shows, cartoons, video games, books, toys, and merchandise."