The way I see it, is that bronies are an extension of furries (lets not argue semantics, there is a LOT of crossover) and furries are notoriously disgusting, perverse, and unsavory. Now, I realize this is a blanket statement. I myself happen to belong to the furry fandom out of association so I just insulted myself as well. I have many working ties that I am a little shameful of. XD
But the point I am trying to make is that when you take the generalized sexual depravity of the furry fandom and apply it to MLP, you get some really twisted stuff. I'm sure everyone has seen the infamous pony plushie with the oddly-specific hole sewn into it. Let's not forget the "life size" Rainbow Dash that sold for thousands on eBay. Only two kinds of people would buy that: the parents of an exceptionally spoiled bratty daughter, or someone whose intentions are to do something unspeakable to it. The furry fandom is HUGE, so it's understandable that they'd have their own slang, like "yiff". Bronies are still very much a small community, and yet they already have "clop" as their equivalent term. There's more than one sexual slang in their community vocabulary as well. For a community that seems to be clean and open, they really picked a lot of weird things to have euphemisms for.
Again, I realize there are exceptions to this rule. Bronies are not unlike fundamental religious wacks, its the misbehaving people that get the most attention, but their actions really do a lot to make the group as a whole look bad. I know that the Humble Bronies raised $10,000 for the summer indie Humble Bundle, and that's incredibly thoughtful of them; but for every group of Humble Bronies, there's about seven groups of people running Tumblrs of people ejaculating on pony toys.
I guess I'll let my Warn status get 20% cooler too.
Hopefully that wasn't too blatantly warn-worthy though; just trying to explain the inappropriate aspects of things and why Skarro probably felt that his picture was humorous and/or relevant.