Their sex, that is:
My hippie parents were always open to talking about the mind-bending wonders of meaningful, loving sex — and how it's the "glue" (ew) of marriage — but I knew little of my mom's sexual and romantic past. I had lots of questions, and her terminal cancer diagnosis a year ago drove home the fact that I might never have them answered. I also know someone whose father passed away before he could ask him some burning sexual questions, namely why his dad's marriage to his mom was largely sexless. Now it will forever be a mystery to him, one that is deeply tied to concerns he has about his own sexual experiences, and it haunts him.
(Photo from the "My Parents Were Awesome" tumblr)