Never been much of a problem for me... we both disclose expected information willingly and openly... and as time goes by, we work up the courage to ask more personal questions and answer them in due time.
You'll never know EVERYTHING about your SO... and that's fine. Some things that have happened in their life do not really contribute to the relationship... and their omission do not hurt the relationship either. It's the things that do contribute to the relationship or can hurt the relationship through omission that are greater concern --- to me anyway.
"The weakest soul, knowing its own weakness, and believing this truth that strength can only be developed by effort and practice, will, thus believing, at once begin to exert itself, and, adding effort to effort, patience to patience, and strength to strength, will never cease to develop, and will at last grow divinely strong."
- James Allen