I think it's a mistake to each as much junk food as I do. I think it's a mistake when other people do it too. If I look at my S/O eating a cheeseburger and go "geez, you really shouldn't eat that junk", acting as though this isn't something I do myself, and am morally superior, I'm a hypocrite. I've failed to hold myself to the same standard. If on the other hand I go "man, we should really work on our diet, we've been letting it slip" or even "man we've becoming a pair of fat disgusting slobs!", I've held us both to the same standard, presented us both as equally flawed for our mistakes, no hypocrisy (though possibly some assholery), even if my S/O has no desire to reduce their cheeseburger consumption.

Now lets look back at the "no sex before marriage crowd". Despite the fact that most of them have done so (and assuming they are unmarried) intend to continue doing so, they will demand it of everyone else (especially women), and even use some fun derogatory terms (again especially towards women). How many have you met who enter the discussion going "look, I, my partner and my children all do this too, we as a group should all try to do better"? How many use the same derogatory terms when describing themselves, their partners, or their children? Would they agree with you if you used those terms for them, their partners, or their children?

Or do they pretend to be morally superior to the people making the same "mistake" they do?

Hypocrisy.