One factor is, horniness of fertile females comes in waves. Just like other mammals, women are more horny when ovulating. But, most months (years) they really don't want to get pregnant. So, even when horny, there is that issue. Birth control is a pita and can fail.
Hormonal birth control can lessen sex drive.
Pregnancy and lactation can also lower sex drive, as can the exhaustion of chasing a toddler and a preschooler all day.
Men's sex drive remains pretty constant from their early teens to around 30.
However. Women begin to hit their sexual peak at around 40, when estrogen starts to lessen. Then their own testosterone comes to the fore. Male's testosterone and energy starts to lessen at that age.
This is why every woman over 40 needs a nice 20-something boytoy.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
Mags (poly, F, 61) loving miss pixi (poly, F, 39) since January 2009, living together since 2013
also loving Punk (42, M) since Oct 2015 (he has recently downgraded us to friends)
"Master," (mono, 34), miss pixi's Dom for 2 years