I think it depends heavily on the context of the image you are viewing, ponies taken straight from the show are not sexy to me, but with the power of fan art anything can be made sexy as seen by the plethora of pictures people have been posting in this thread. However, I will say that hypothetically if ponies did exist and humans interacted with them I could see one advantage to being in a sexual relationship with a mare compared to a woman, and that is ponies are so biologically different from humans you wouldn't have to worry about having children with them which I guess could be an advantage or disadvantage depending on the person. However, if you somehow managed to break the basic laws of nature and had a child with a pony you would end up having a centaur, which are easily the most badass creatures in the (fictional) known universe. I mean just look at this picture
Who wouldn't want that as their child?!
In all seriousness it all depends on the circumstances, taken directly from the show they are not sexy, but with the right conditions they can indeed be sexy.