Androgens are responsible for libido in both sexes!
Also, can't be estrogen because they're on HRT and vagina is moist and rugated, indicating adequate estrogen
this one is tough but this is what I am thinking. If you loose your Ovaries you loose your major source of estrogen. Once your replace that orally, you will have excess estrogen floating in the body. that will go shut down your HPA axis, so your other androgens will now be decreased --> low libido