I'd see this like:

"So, you're tellin' Bronies are horse fuckers?
Clearly, everyone wants to have sex with everything one likes.

"horse f###er" implies its bestiality, which it may or may not be.
Why "may not be"? Well, "best" implies "beast": an unintelligent form of life. "beast" would imply the creature cannot speak, build, or form advanced communities (such as towns, cities,advanced governments, etc.).
Now, C.S. Lewis's "Chronicles of Narnia" does feature what he calls "talking beasts". They can speak, but as Qui-Gon Jinn in "The Phantom Menace" states: "The ability to speak does not make you intelligent". As such, though those creatures can speak, they lack the other abilities that would otherwise make them intelligent (aside from, say, centaurs & fauns in the series).
The MLP ponies do carry the ability to speak, build cities and citadels, have advanced form of government (an absolute-monarchial position held by Celestia) and form advanced social communities, with an economy. In short, they are intelligent. Therefore, the term "beast" may not be applicable.
The main issue is that in Real Life, there are no creatures (that we know of now) that are intelligent as the homo sapiens. Therefore, the prospect of inter-intelligent life mating is not particularly thought of. The nature of the equines in the show makes it the target of "beast" though if they existed in real life they may not be.