I think this sums it up excellently. It's a bit more complicated than that but that's probably the easiest, quickest way to put it for people to understand.
(well, y'know, removing the wife part -- I don't think most of us are in such a situation anyway... but we have our other reasons.)
In the cartoon? They're freaking adorable. There are subtle things like I've mentioned like the eyes / expressiveness, legs/flanks, voices, overall design. And certain things in certain episodes that would make certain creepy idiots like me go "hmmmm", like Bathrobe Rarity, Whiny Rarity, Burlesque Pinkie, First-Person-View Rarity from Secret of my Excess, Fleur-de-Lis and her posing, etc etc. But mostly no, they don't usually make most of us feel that way; and when I watch the cartoon, especially when I watch new episodes, I like to kinda 'turn into a kid again' and watch them with the glee and innocence of an age before I even cared about such things at all. In fact, in the beginning when I first got into FiM it kinda 'cleansed' me and I didn't care about any kind of sexualized cartoon stuff/furry art/ESPECIALLY pony art like that, for several months (even after some of those episodes I mentioned aired). It really took a long time of 'thinking about it' and being slowly influenced by the increasing amounts of increasingly well-made and subtly juicy art that kept slipping through the cracks. And about that...
In fanart? They can range from... anything to anything else depending on the mood the artist is trying to convey / their skill level / the touches with which the piece was crafted and so on, but yeah... anyone who denies that they CAN be made sexy, be it in full-pony form, anthro form, humanized, or whatever, is just kinda in denial. Even if FOR YOU PERSONALLY they can never be made sexy no matter what, by any artist (and there's NOTHING wrong with that, of course!!!), for most of us -- for most of us individuals who find ourselves online and in communities centered around cartoons and who look at a lot of art of stuff like that in the first place -- it's certainly very likely that somehow, some way, through some amount of excellent art that varies from person to person, we can and will get slowly 'tilted toward' being okay with that stuff, even kinda liking it, and eventually probably *really* liking it. I only hope those of you who personally DON'T like the stuff (and never will, even) can realize that and understand that it doesn't make us bad people or make us 'ruin the community' or anything. The only people who're bad, again, are those that try to force it on others or leave things untagged / out in the open so kids and people who DON'T wanna see it might accidentally run into it. But most of us? We'd never EVER bother someone who's not into it about it. Don't hate, tolerate!