This is my feeling as well. I also think that Sherlock is extremely comfortable with himself, and I don't know if he would be if he'd had a childhood where he was told he was wrong and a freak all the time.

Abusive homes seem to be a trope in fanfic, though. A lot of people give John an abusive childhood, which is a bit more reasonable to me, but my own John grew-up in a more 'inability to communicate' household, than an abusive one'. I also picture Harry taking up a lot of attention, and John being left to his own devices to cope.