"Diana… In another second or so, I'll feel like kissing you -- And we both know I shouldn't! "
Ah, how things change over four decades. The above quote comes from Superman in World's Finest #204 explaining that - despite his natural attraction to Wonder Woman - the two of them could never progress past friendship (Admittedly, the "why" is somewhat hidden in his statement; let's put it down to his heart belonging to Lois Lane at the time, shall we?). He delivers the line at the end of a story that saw the two Justice Leaguers team up to fight crime after being set up on a blind date together by a computer dating service, with Wonder Woman telling him, "Funny… we've been acquaintances for years -- Yet we've never really gotten to be friends! Maybe we should do something about that!" (In case you think that Superman's romantic rebuttal comes out of nowhere, considering Diana is just asking for friendship, it seems that he was reading between the lines correctly. In response to the "I shouldn't!," Diana says "Right you are -- Darn it!" Apparently, romance was all subtext in the '70s).
The idea of pairing Superman and Wonder Woman feels like a somewhat natural one on a basic level: He's the world's greatest superhero, she's the world's greatest super heroine, so why shouldn't they be a couple? But, for the most part, it's an idea that was left untouched for the first few decades of each character's history, "imaginary stories" aside (The two actually married in a dream sequence that appeared in 1983's Wonder Woman #300) - In part because inter-universe continuity wasn't such a big deal until Marvel Comics came along in the 1960s, and in part because both Superman and Wonder Woman were spoken for (by Lois and Steve Trevor, respectively).
Plus, these were comics and characters aimed squarely at a kid demographic, and such soap opera romances didn't really feature in the formula at the time. But the issue World's Finest excuse that the two shouldn't be together just because started to fall by the wayside in more recent years, as audiences became less willing to put up with the status quo and creators more happy to push and pull at accepted ideas in the name of a good story - or, at least, a shocking hook to make you pay attention and pick up the book.
Take, for example, the well-known kiss between the characters in Action Comics #600, from 1988. "Superman's Romance With Wonder Woman!," as the cover promised, turned out to last exactly one kiss, and even then, it wasn't a kiss exactly reciprocated by Diana.
Superman's hormones - and, apparently, his gullibility when it came to some well-placed but entirely fictitious press - had gotten the better of him, it turned out, but at least he made up for it by helping Wonder Woman face off against Darkseid and save the mythical gods that she worshipped. That initial post-Crisis team-up of the two characters (Legends mini-series aside) set the tone for the relationship between the two for the entirety of their relationship over the next two and a half decades: Friends, and just friends, despite occasional "what if"s on the side of one or the other.
Take, for example, 1999's Action Comics #761, in which Superman and Wonder Woman ended up fighting side-by-side in an alternate-dimensional war that lasted one thousand years. Before what was believed to be the final battle, Diana makes overtures to Clark ("This night --" "--is our last night together… yes") that he turns down. "I… I can't, Diana," he tells her. "Silly, right? Even if she is… gone … A thousand years, and another world past… Lois is still the only one."
Other Supermen, however, don't have that same will power when it comes to saying no to an Amazon Warrior Princess. 1996's Kingdom Come , after all, includes Diana and Clark getting together, with the 1999 follow-up event The Kingdom even seeing the birth of the couple's son, Jonathan. The offspring of a Kent/Prince pairing was also a centerpiece of the 2001 Dark Knight Strikes Again series, with Lara - the somewhat morally-challenged daughter of Superman and Wonder Woman who doesn't really understand why superheroes should stand up for the common man - playing a central role to the series, which also features a has-to-be-seen-to-be-believed sex scene between Princess Diana and the Man of Steel (The early days of that relationship was later seen in All-Star Batman and Robin , in 2005, with suitably overwrought captions as the two characters kiss: "The thunderbolts. They seem to stab them both. The world only seems to quake").
The pairing of the characters in Justice League - and, presumably, their own solo titles afterwards - is a milestone of sorts, then; the first time that the "main" universe's versions of Superman and Wonder Woman have become involved and meant it. Geoff Johns calls the relationship the new status quo, but that may simply be misdirection… There's a Trinity War on the horizon, after all, and the two characters will be hooking up at an emotionally turbulent time for both of them as a result of the "Villain's Journey" story arc; not exactly the best time to enter into a long-term relationship, perhaps.
But, a year after DC demonstrated a willingness - one could even say a desire - to shake up expectations of its most familiar characters with the whole New 52 relaunch, playing matchmaker with these two characters without an immediate reversal or get-out clause is definitely a good way to remind people that this is a DC Universe where anything can happen - even if it's not something that everyone wants to see.