We don't. Seriously. That's a 4chan thing not a brony thing. Most of us on the broader internet--at least half of us on YouTube--are _well aware_ it's a Youth-rated show and that this crap is well, crap and completely inappropriate.
As for 4channel? They have a longstanding track record of being perverted about their cartoons, particularly if the characters are anthropomorphized at all (read: passably intelligent and human-like). They'll make excuses of the "oh but they're a sentient species" sort, but really now....
I'm thinking half of it is trolling on their part, and that the other half might well be confusing attraction to _characters_ played by the _actresses_ with feelings towards the literal ponies.
But yeah, for most of us _non-4chan_ people it's just a simple, innocent cartoon. A sugar rush. Nothing more. Entirely platonic.
Really, check out 4chan's track record on Pokemon. Much the same deal, only worse.