There's a joke that the most significant substance reducing libido or sex drive in women is wedding cake.
A German researcher, Dr. Dietrich Klusmann from Hamburg-Eppendorf University Hospital, interviewed 530 men and women about their sex lives.
He found that 60% of 30-year-old women wanted sex "often" at the beginning of a relationship; but within four years of the relationship this figure fell to under 50%, and after 20 years it dropped to about 20%. In general, the longer a woman was in a committed relationship, the less she desired sex.
However, the proportion of men wanting sex "often" remained at between 60-80%, regardless of how long they had been in a relationship.
Dr. Klusmann attributed these changes in frequency of desire in men and women as due to evolution. There may be other factors, though. Is this a culture thing, or an age thing?
I will admit that I find this a little unsettling.