I could be that when you were very young you happened to see some of the porn that perhaps your family members were watching. A young child may not exactly remember seeing something like that, but might retain what they saw and that could begin a deep fear as a child would not understand it if they happened to see something like that.

Children "do" discover themselves and a child can actually discover this at surprisingly young ages too. Most children tend to be curious if other children experience this as well. Often parents never talk to their child about it so the child simply just doesn't know what it means. It is actually not unusual for a child to explore this and at the same time have trepedations about it.

It's important to understand that you are only human and human beings discover this and it's just something that's part of "normal human function". Masturbation is something a lot of children do, it's not evil or bad or dirty, it's just "human nature". It's important that you make sure you don't encourage yourself to feel guilty for having a normal human desire that you explored.