No one has said that there can't be more than one black character (or female character). When it comes to the diversity issue, again, I have no problem with including more black or female characters. The only thing I've said, is I don't agree with replacing the big stars just to add more diversity, thus, forcing them in just to be diverse.
Some people have different opinions about which characters they like best, to be expected not everyone is the same, but to most fans and the general audience, Batman, Superman, Wonder Woman, Hal Jordan as Green Lantern, Flash and Aquaman are the stars of not only the Justice League but DC Comics in general (hero wise). Those are the top guns of DC to most people, now obviously Flash, Lantern and Aquaman aren't on the same level as Batman, Superman and Wonder Woman but those 6 are the most time honored and popular characters, generally speaking. The majority of fans and the general audience want to see those characters the most. Because they are the most popular. Like it or not, Hal Jordan is the most recognizable Green Lantern. I'm not saying that there isn't a strong fanbase for John Stewart, or that there isn't a big group of fans that like John better than Hal but Hal is the most popular Green Lantern, he just is. And those characters all happen to be "white", it's nobodies fault that the most popular characters are white, that's just how they were written in the beginning and those are the characters that gained the biggest fanbases, the characters that the fans made the most popular by buying their products more than other characters.
Martian Manhunter, John Stewart and Cyborg just aren't as popular overall, they all have their own fanbase but they just aren't on that level with the main 6. Actually, in recent years I would say Green Arrow has become more popular than those 3 as well, another white character. More fans connect with those characters for whatever reason (IMO it has nothing to do with race and everything to do with how interesting the characters are).
Think of it in terms of Boxing, yes, there are alot of people that like the undercard boxers but at the end of day, everyone wants to see the main event. Stewart, Cyborg and even Martian Manhunter are just undercard characters, not because they are black but because they are not as popular as the other 6 are.
So for me, and the majority of fans/general audience (even if it's a slim majority advantage, which I don't think it is) those 6 have to be there first and foremost. You could put Manhunter, Cyborg and John Stewart (and an extra female character or two as well since that's another part of the diversity debate) in the movie and I would have zero problem with it. But everyone wants the big guns in the movie and unfortunately for those that want more diversity, the most popular characters just happen to be white. Personally I don't care what color the characters are, I just want the most interesting characters. The most interesting black characters for me are Blade and Storm, but they're not DC characters, I like both of those characters alot. I just don't find the black DC characters that interesting.