With both supporters and opponents of the idea of pairing DC's two most powerful heroes chiming in passionately, we started thinking about whether this could actually be sustained as a long-term story...and ultimately we decided the answer was no.
Why not? Well...
Action Comics #600
The most iconic hook-up the pair have ever had was in Action Comics #600, where they were depicted on the cover in a passionate kiss not unlike the one we see on Justice League #12.
Inside, though? Not so exciting. Essentially, after the moment of passion subsided, the pair were left awkwardly looking at their red booties, talking about how they needed to walk the dog or wash their hair or something. Someone online described it as the realization that it was a bit too much like kissing your brother or sister, and that's a fair assessment of how it was depicted.
In the world of the New 52, you can get away with this a little more easily and believably as a short-term storyline; after all, there aren't literally decades of stories behind us that define the characters' relationship together. Still, it's impossible to ignore the fact that even in this context, they've been friends for five years, during which time they've no doubt shared details of one another's personal lives and so each of them knows about the on-again, off-again long-term love interest that the other has on the side.
Practicality, Characterization and Gender Roles
We've talked about this before, but it's important enough to raise it again. Either in reality or at least in the minds of a great many people, the shift from being a headliner in her own right to being Superman's leading lady will ultimately hurt Wonder Woman as a character and, arguably more importantly to DC, as a brand.
Superman and Batman are character that steamroll everything they come into contact with--Dick Grayson might try to get away from his past at Robin, but when Bruce comes a-callin', Dick is ready to step up and take on those pointy ears and that scalloped cape in a heartbeat.
In spite of the best intentions of writers for years, it's been impossible for anyone except Kirby (with his Jimmy Olsen stories) to really separate those closest to Superman and Batman from being "just another face" in the cities controlled by those heroes. That was a central theme in books like Gotham Central and has led to stories where Superboy or Steel or Nightwing or whoever decide that the only way to be their "own man" is to pull up stakes and run away.
This may seem like a cheat, because it's not really about Superman and Wonder Woman, per se, but if they were to stay together long-term, wouldn't they just morph into "mom and dad," with the Justice League being subordinate to the dominant presence of the world's finest couple? It seems as though the whole thing would change the team's chemistry in a way that you can't go back from.
Meanwhile, there's also got to be a sense that the team can't trust the pair of them to lead as impartially or to have everyone's back in the same way during a fight if they're making invulnerable little babies on the side. It's the old argument against relationships within a military unit: whether it's true or not, the perception is going to be that you aren't giving everyone the same level of loyalty and protection that you are the person you're going home to.
He's not exactly a defining element of the post-Crisis on Infinite Earths take on Wonder Woman, but no matter how many times they try and shake him, Steve Trevor always makes his way back into her life. He's Wonder Woman's man at the end of the day, unless Geoff Johns has his arm ripped off or something.
The fact that he's currently acting as a liaison actually seems the most likely thing to turn Diana's relationship with the Man of Steel on its ear, as the guy she's "supposed to" end up with is there all the time, observing them and making those observations vocal at potentially key intervals.
Of course, the biggest, most obvious answer this relationship can't work in the long term is simple: Superman has a date with destiny. Steve Trevor may be Wonder Woman's love interest to those who have been paying attention, but you don't even need to have ever seen or read a Superman story in order to know that Superman's Girl Friend is Lois Lane. It's part of the DNA of the character, and they can't keep her on the sidelines for too long.
The dynamic that Superman has with Lois--that she's extremely vulnerable as a normal human working in this world full of super-powered maniacs, but that she doesn't ask for help because she's one of DC's strongest characters in a lot of ways--is key to the relationship he has with Superman. If he couldn't change the course of mighty rivers and bend steel in his bare hands, Lois could take him in a fight--and even with his powers, it's six to five and pick 'em. Lois doesn't need to be Zeus's daughter; she's formidable enough--and perfect for Superman--on her own.