I would like to talk about something that I so far have not seen described here. I would like to preface this post by saying that I tend to seek naturalist explanations for phenomena in the world. Perhaps Marnia can add a more spiritual commentary. I'm certain that I'm not the first to think of this and here is an excellent place to discuss it.
Marnia talk about a sense of separation between male and female in the world being a source of angst. I like the analogy that male and female put together are like to parts of an electrical circuit with each needing the other to be complete.
It seems to me that human beings have an innate sense of lack in their lives. It seems that we are constantly trying to fill that void with the opposite sex, and this is where Marnia's ideas come in to help us understand how to experience lasting satisfaction. I believe that everyone has experienced this sense of lack, and many of us have forgotten that there was a time before we felt that sense of lack.
Think back to when you were a child on Christmas morning. I remember a time when it felt just awesome to open up all those boxes filled with gifts and toys without a care in the world. I remember coming home from school with not much more on my mind than what video game to play once my homework was done. As a child, it seemed that there was no need to seek relationships with the opposite sex to fill a perceived void in my life, and I assume that many, if not most other people feel likewise.
It seems that at some point in time, a perceived sense of lack sets in. If Marnia is right, then biology uses this sense of lack to drive us from partner to partner, and biology's script is rather painful. I think that this sense of lack, the feeling of something being wrong setting in is what is really the source of the feeling that people are talking about when they use the words "Innocence Lost".