One: A love Story by Richard Bach
There are several paragraphs/ maybe a chapter in One:A love Story by Richard Bach that deals with this.
Bach is describing his love for his wife, Leslie, and describes parallel lives where her experiences are very different, (i.e. a drug addict or a prostitute) and questions if he would love her the same, if she was cynical, if she was jaded, if she didn't love him. Interesting discussion of unconditional love.
That's another book I need to re-read. I'm delaying reading the last few pages of The Moon Is a Harsh Mistress... it went too fast this time.
(This is how I remember the passage...I don't have the book for reference)