I think most Bronies do it just to be weird. As geek culture has taken over pop culture it’s harder and harder to be an outlier, and some people have a large drive to be different. Teen to adult male watching a cartoon target marketed at 6 year old girls, that’s different.
I think this is the explanation too. But they're still a bunch of sick freaks.
A long time ago, I was advised by a student, a college junior who took an upper level undergrad course in combinatorics from me, who found out that I liked (at least some) anime, "You have to see Cardcaptor Sakura, yeah, yeah, I know, but trust me, you have to see it." (CCS as it is known among otaku -- the Japanese word imported into English to denote "anime geeks" -- seems to have a target audience of six to ten year old girls.) I ignored his advice for years until I found that xxxHolic which I loved (about poor bloke working for a witch who runs a wish granting shop -- an enterprise which is a priori sinister in light of the Buddha's insight into the relationship between desire and suffering) was set in the same "mythos" as CCS. I would not give a "must see" recommendation for CCS to anyone older than ten unless and until I knew the person asking for recommendations had developed a Japanese patience for slowly developing plots. The episodic plots are trite, enjoyable only by the target age-group, but over the whole course of the (I think four season long) show there are two long-term story arcs that are very satisfying even for adult viewers.
I haven't clue whether the new-issue My Little Pony series has that sort of structure -- if it does, I'm inclined to pardon the wierdness of the "Bronies"; if it doesn't, I think the standard advice "Get a life!" is in order.
If they're really that desperate for attention, couldn't they find some way of being different that doesn't give off pedophilia vibes? For instance, I think that walking around town in Victorian attire (top hat, cane, cloak, muttonchops) would give them the attention they want and the satisfaction of being "different", while not setting off the same alarms that dressing up in pastel-colored pony costumes inevitably do.
Some cartoon writers throw the occasional ‘adult’ reference in there to ‘throw a bone’ (as it were) to those adults that they know are watching their show. They watch because the kids ask them to.
One show that I know throws in the adult references regularly is ‘VeggieTales’. On an episode from a couple years ago, they did a show that was sci-fi themed. In the course of the show, they threw in references to at least 7 sci-fi series or movies that only adults would get.
As for the bronies....well....it’s one thing to watch MLP with your kids or grandkids-nothing wrong with that at all. For a ‘grown’ man to deliberately watch it...that’s something that gives me the creeps.