I too am just starting to experience this problem.... but it has not always been an issue, just with in the last year or so of our 23 year relationship together - 20 years married.
Knowing that SEX & SEXUAL PLEASURE is 90% mental and only 10% physical - I have to say that I am now on the verge of becoming anorexic in the bed (for sex), which by the way is a disorder of its own.
LoVe,
Rhapsody -