Hmmm....well, no, I don't think it's something you are born knowing.
I think a lot depends on what you are brought up to know and probably to some extent from what age. You're saying that you knew, but I'm still interested to know at what age it occurred to you that you knew? If it is something you'd grown up with, do you think it would have been so clear cut for you?
I think children can pick up on, even very small non-verbal cues from their abuser though that something isn't quite right about it sometimes, even if it's being presented to them as okay and normal, but that doesn't necessarily happen in all cases.
If all the information you are receiving as a child comes from your primary carers and you don't get a lot of other input, it's not always until you start having more contact with the wider world that you start getting different cues about things.