I find them two different things... I'll admit, the human sexual characteristics are a strong thing. It's really a level, it depends where you personally draw the line, girls with cat ears and little paw boots, girl with the fur of a cat and cat like parts, girl that is more cat than human, and at the extreme, cat.
I mean it's sort of natural, there are studies where when pheromones or courtship behaviors of a species are preformed by another species, often the audience species will respond sexually despite the performer being a different species.
Sometimes animals when sexually aroused will become sexual with same sex of its species or completely different species.
We are preprogrammed to find breasts and the human facial structure and ******s to be sexual, as well as the female form, so when things are changed around, it can still be sexually appealing.
Now affiliations and preferences to certain animals are common. I like reptiles and have various species I find "cool". I find ways in which I and some organisms relate, its natural, a chimpanzee being killed is for that reason more distasteful to us than a giant sea turtle.
That's how some semi furry stuff is explained.
Bronies however have less to do with that and more to do with the Internet and the show, and things such as popularity, shock, animation, ability to be made into memes, perversion of characters and transformations of stock children show characters into mature risqué characters that are ironic in comparison.
Aka, the queen or princess celestia being a frat boy or dictator.
The sporty tomboy being a lesbian sexual fiend, the shy peaceful introverted character/ happygolucky partier character being a mutilating closet serial killer.
It all ties into the internets love for irony, sex, humor, and perversion of content.
No one actually learns or cares about the show really, it's market is young girls. It's basis is to market the my little pony franchise by using its characters and background as conduits through which to teach lessons and virtues and ideas to young girls.
Really there is a line that needs to be drawn, between the people that laugh at the memes and understands the premise of it, and the people that spend money on my little pony stuff, watch the show, are on the chat boards and attend events meant for young girls and their chaperone parents.
And drawn the line between the people that are aroused by some "furry" porn, like animals, relate to some, and the people that are into beastiality, find animals attractive, wear suits, attend conventions, and believe in furry stuff.
Really, everyone has their kinks, and I can personally say that photos of lizard girls and naked women with cat ears and tails, and memes from a children's show, are both a hell of a lot better than some of the stuff on the internet.
Also, my post is much better than yiff yiff yiff yiff yiff, so **** off if you are going to try that Tldr crap.