I think a good deal of it has to do with puberty. If you’re attracted to prepubescent kids, pretty much everyone agrees that you’re a sick pervert and probably should be locked away. If on the other hand you are attracted to teenage girls, well, people understand that. They don’t condone it or like it, but I doubt there many men who haven’t found themselves looking at what they thought was an attractive adult woman only to find out that she was underage. Now the vast majority of us can control our sexual impulses at least well enough to avoid coming on to 15 year olds, but we can understand where those impulses come from.