Great words and advice, @nomad-magus! I have a lot to say regarding this matter, but let me sum it up in a few sentences.
The past does not matter, as long as it doesn't matter to you nor your partner. The only time the past matters is when you always look back to it, as if missing the person you were with, and neglecting the person you are with. That connotes a problem that you both should fix - and usually the fix it requires is separation.
But, let me put an exception to this. Sometimes, the past does matter to avoid and correct mistakes that can happen at the present.
So, if your partner genuinely opens up his/her past to you, treasure him/her. Recognize his/her efforts to be honest with you. Because honesty is the start and the anchor of a fruitful long-term relationship.