Just start it.
It's an exceedingly important discussion, and not having it is almost sure to wreak havoc somewhere down the line.
My ex and I not only had that discussion before we were married, but I actually preached a sermon on that topic at my wedding.
Maybe I'm from another planet or something (and I actually say that to people all of the time in real life), but I believe that people need to be open and honest with each other...come what may.
I mean, what's the alternative?
Closed and dishonest?
If you're considering spending the rest of life together with somebody, then there needs to be open and honest communication between the two of you.
I still recall something that my ex said to me very early on in our marriage. We were lying next to each other in bed, and she turned to me and asked "Do I need to tell you things now that we're married?"
Her question was unsolicited by anything that I had said or done, but, truth be told, she never really told me anything about herself. I found out a lot of things along the way, but I pretty much didn't know anything about her before we were married. I definitely didn't know that her father had sexually molested her repeatedly as a child between the ages of 5 and 6 years old. I discovered that through a series of events about 1 month into our marriage, and, believe me, that past trauma greatly affected our marriage in more ways than one.