I can't say how much of a problem it is, but then I suspect nobody can.
As a parent (and yes, as stated already, I do limit internet access) I know that most parents definitely wouldn't want these things to be viewed early in life - I have no problem with adults viewing whatever they want, although I would suggest that people who constantly search for the NSFL content are possibly not well adjusted.
I don't claim to know the answer - I just get the impression that these options are always pushed away due to 'freedom' but there is little alternative offered to help society maintain a healthy mental balance.