As someone who didn't cut his hair from age 30-35, there's an unexpected sense of masculine power that comes from long hair. Part of the reason I did it was to challenge my own ingrained notions of man=USMC jarhead haircut. I typically just wore mine back in a long straight ponytail.
And I agree with Timbales, there's no need to prefix "man" to it. Nobody calls it a man-ponytail. It's just a ponytail. Calling it a man-bun, from my perspective, sounds like an attempt to hold at bay the femininity of wearing your hair in a "bun". If you want to wear your hair in a bun, put it up in a bun and be proud of it. To turn around and call it a man-bun just seems a little bit like chickening out halfway. You have a bun on your head, period. Own it and enjoy it.
One aftereffect I've found interesting is, even though my hair has been very short the past three years, I've noticed that there now are long-haired men I find attractive, and I am certain I wouldn't have found them attractive 10 years ago (I had always had a fairly straightforward buzzcut). Having run the experiment of long hair, I can now see through the hair to the man underneath without my brain automatically emasculating my perception of him.
Our ideas of what is masculine or feminine or attractive are partially instinctive but I think are so much more heavily influenced by childhood cultural biases than most people realize. Consider this thought experiment: You are a caucasian anthropologist who has always been predominantly attracted to other caucasian men. For postdoc research you go spend five years living in mountain villages deep in rural Nepal with no access to the Internet. There are certain to be some gay men in Nepal -- if they've never been exposed to the modern idealized white/male/masculine/athletic/wealthy advertising industry, don't you think they experience attraction to other Nepalese men around them? And in those five years, do you think it's possible your brain would somewhat rewire itself in such a way that one or two of the locals might even become attractive to you as you internalize local notions of strength, masculinity, leadership, etc. and start to see manhood the way they see it?