My concern with this thread, and the reason I keep posting is this:
A lot of folks have posted opinions, anthropormorphic ideas about horse's behavior and interior psychology, some moderately funny ideas about homosexuality in animals and some pointless speculation.
I do not pretend to have done extensive research into homosexuality in animals. What I do have is extensive experience with horses, period, including a long period of time working with breeding stock. Yes, I've seen geldings mount mares. Yes, I've seen geldings mount other geldings. I've seen a lot of hormonally produced bizarre behavior, too long to describe in this thread.
BUT, THE ONE AND ONLY TIME I saw a mare exhibit stallion-like sexual behavior, she had an ovarian tumor that eventually killed her.
So, can we please hold off on the rest of distracting speculation and opinion until the OP gets this mare checked by a vet? If the mare does not have an endocrine problem; please feel free to resume the entertaining conversation.
But on the very real chance this mare does have an endocrine problem, can we please hold off on tangents and sidebars that might distract the OP from a very real medical problem?