I know that some children can get sexually excited someway, and abuse is not the one and only reason why. I remember myself at the age of two lying on the bed and slipping my hand into my diaper to pinch my butt, this was giving me some kind of sexual pleasure.
Some psychologist suggests that this is a common issue between those babies and toddlers who suffer a lack of snuggle and attachment from their parents, as it happens for abandoned children. Breastfed babies are much less likely to spank the monkey.
This makes sense to me since my mother didn't nurse me, she had some kind of reject of her womanliness and I remember feeling I was troublesome to her when looking for her cuddle .
Hope this helps