I've seen plenty of films that were set (and made) in the '60s that didn't have those two particular people referenced in them.
I don't see a problem with at least referencing those pivotal and influential figures in the film.
True, given that civil rights would be a concern for the mutant characters, you'd have a point there. However, Singer didn't have to set the film in the '60s, did he? I mean, The Last Stand showed Xavier taking on Jean as a student 20 years before the time frame of the movie, which given its "not-too-distant future" setting, would suggest 1990 or so.
We can't be specific. "Suggesting" 1990 is exactly that, and judging by the cars and clothing both Xavier and Erik wear in that scene, it would suggest 1970's or 1980's to me. However, I've been told Vaughn and Singer are flat-out ignoring elements of
X-Men: The Last Stand. For example, Xavier gets crippled in this film so of course it contradicts with the events in
X3, where he's shown to be standing. I'm sure some will have problems with that, but I couldn't care less.
If anything, the '60s seems too early a time frame for the film if it's meant to be in the same continuity as the others. It feels like Singer's being revisionist and deliberately choosing that time frame both because it's when the comic debuted and because of its civil-rights resonances.
I think if anything it'll have similar continuity with the first two films directed by Singer. I think Singer and Vaughn are telling the story they want to tell -- and I think they are right to pick and choose what elements they wish in order to successfully tell that story, whatever it may be. I think some fans are beholden to certain things like exact continuity but I think those are trivial instances and in the bigger, grander scheme of things so long as the film is good I couldn't care less about the exact nature of continuity and whether it is identical to what we've seen before.
Yes, I'm sure the retort will be, "Why not reboot if you're not going to pay attention to detail?", and I think Singer is a very deliberate filmmaker and will pay attention to detail to an extent, but also give room to be able to tell the story he wants. Also, I have no problem with disregarding the events of the last two
X-Men films anyway. They sucked.