In case it isn't said I just want to thank Mark for his candidness. I also agree with him.
Another thought (in addition to age & hormones) would be that dropping weight changes the way we see ourselves. How many people refer to themselves as getting "sexier" as they lose weight. For many people the idea of seeing themselves as sexy is both exciting and frightening. I guess I'm saying there can be a number of psychological aspects associated with libido loss. Perhaps you could find some ways to make the intimacy more appealing- for example romantic dinner or back massage preceding. My 2 cents.