They would've a decade or two ago (Still can't believe the 90s is now "two decades ago"...), but by this point it's become pretty mainstream for men to not have to only like SHOOT BLOODSTORM XTREME BOOMFEST: THE EXPLODENING. I guess you COULD say they pose a challenge to traditional masculine values, but those values are outdated enough it's almost like saying the current government of, say, the UK poses a challenge to traditional feudalism (Obviously using a bit of hyperbole here, but I trust people will get where I'm coming from).
That, and (some) bronies have become the equivalent to hipsters, just butting in and flaunting their love of MLP. I have no trouble whatsoever with people being bronies, talking about MLP, having a pony avatar, etc... But I've had very unpleasant cases of bronies completely derailing things that have nothing to do with MLP, like for instance, there was a whole lot of them on a certain Bastion video on YouTube because it had been linked to by some MLP fanfic as background music. The comment section was absolutely flooded by people going on about MLP, and when I posted a comment about Bastion (Not responding in any way to any MLP comment, not telling them to cut it off, just quoting two lines from Bastion), I got no less than six responses being angry at me for "ruining the MLP conversation".
I guess that was a bit long-winded but my case is: You can't really issue a reasonable challenge to a set of established values unless you behave reasonably maturely about it. I know that the majority of MLP fans are reasonable people who'll limit their discussion of MLP on non-MLP related things to a reference and an avatar, but it's that minority who acts like they're some godly caste that makes the entirety of the brony community look like annoying, obnoxious and inconsiderate people. To be completely honest, kinda reminds me of the "PC master race" bunch, making every PC gamer look like an elitist asshole. As long as those people are around, bronies will be seen as immature and annoying, which impedes them from making any real challenge to traditional masculine values.