I have never done it..but I have a vanilla lesbian partner in real life...so I am not sure how often it's really done, but I imagine more than you would think and less than porn would potray. If it is a hard limit than it should never be broken. Never. But if it's a soft limit *like it would be for me* than slowing working up to it would be the way to really handle things. Starting with gentle finger pushes and then working your way up to small then larger plugs...then a cock would be the way to go in my thoughts anyway. If it is a soft limit and your Master wishes it...then I personally would go out of my way to train myself to love it, for pleasing him would please me in the long run.
NO it shoud not be used as punishment...particularly before you are very very used to it. Otherwise it would be very close to rape....and yes that is a strong word for it, but doing anything that is beyond what YOU have given permission to do is rape...Master or not. But rape I mean ONLY if it a hard limit!
But for example, if my Mistress PRO DOM asked me to push a soft limit on anal *a bigger plug for example, I would to please her. Personally I am curious about anal and would not want it ruined by making it a punishment.