There's much debate on this issue, and ideally you would have no secrets from your partner, but I think some things need to be kept to yourself. I don't need to know your bodily functions, or how your ex was in bed. Thanks, but no thanks, babe.
Never tell your partner how many sexual partners you've had. They think they want to know, they usually ask too. But actually knowing serves no good purpose.
Journals. My partner writes in one every night before bed and I have no idea what any of it says. If she wants to share with me she can. Those are her private thoughts and feelings until she decides differently. Same goes for me.