The crew of the Enterprise is in no way meant to be representative of future humanity -- not at all. They are representative of the American Sphere of Influence today. Their attitudes, their manner of speaking, their ways of reacting, even their ways of making love, are all contemporary.
We have met the Enterprise -- and they are us.
The crew of the Enterprise is twentieth-century America in space.
And -- although it takes a bit of justification -- that's the way it has to be.
Remember, this is drama we are talking about, as well as American television. It has to make money.
That means it has to have appeal and that its characters must be attractive and interesting -- an audience has to be able to relate to them. Even if the show is alien to the audience's experience, the characters have to be recognizable.