High levels of estrogen in a women's cycle lead to increase desire for sex.
Its a biological thing, a women needs to mate when they are fertile and nature has made sure
that this will happen

.
If your HRT levels of estrogen are high enough, you'll get an increased libido all the time
(since your level does not fluctuate). I saw that myself when I increased my estrogen level.
We are animals, at least I am, after all

At menopause, women often see their libido flagging because their estrogen levels go way down.
Even in men, estrogen (converted through aromatase from testosterone) plays a role in libido.
A man needs both T and E to be really aroused (interesting isn't it :-).
Estrogen has many role in a male body, most man are more of a "women" than they know