Ok - I will answer here, being that I am middle age - 40 and have been married for 21 years (come this Friday - ).

I would say that NOT wanting physical love (sex) any more would probably have more to due with an emotional reason that was never resolved, hence sex is no longer needed or wanted - because for most females we bond to sex through emotional needs several days before the actual act has even taken place.... so to destroy the emotional love / bond in any way is to take away her sexual need over time.

Plus..... there is always the obvious reasons:
stress, being over worked, medical problems, medicines, small children, depression, resentment, anger, self hate.

I personally know that when I go through periods of not being interested in sex it is usually due to me not feeling loved or wanted in the emotional realm by my husband - I need to be loved and cherished out side of the bed room to enable me to want to give of myself in the bed room.

LoVe,
Rhapsody -