Lots of things trigger small children and we generally don’t remember what happened up to around 4yrs old unless it was a very traumatic event that stood out. Media, could have watched some adult tv show at a too young age would be most innocent explanation. Could have been a sexual exploration 'game' with older kids and it might have been abuse by an adult. I am hesitant to call it the latter one for in general those don’t just stop and last years, you would surely have had some awareness later on. But you are right, the thoughts don’t come from nowhere and you are not at fault for having it at such a young age. Mine was my father’s adult magazines, an age where corporal discipline was still prevalent in schools and our media, and being molested by a kid that lived with us and his group of friends when I was 5, them 7 or 8. All that meant these type of thoughts from a young age. I will say if you don’t remember being abused before the age of 12 you can blame media. Anyhow, accept you have it and try not to harm yourself or others with it. Good luck.