I'm late to this topic, but I thought I'd weigh in anyway.
1. The provocatively named "man-spreading" is obviously the complete opposite of a hot button issue. It's an annoying social irritant with a moderate gender bent to it that generally manifests itself as one of the 5,000 ways people are unnecessarily rude on transit. It is not a boiling front on the gender war, despite how some of the gentlemen posting in this thread are treating it.
2. I'm a tall guy with long legs, so I will occasionally spread them farther than shoulder width if it prevents me crushing them into the seat ahead of me. Pressing my knees into a metal seat-back is extremely uncomfortable. That said, if someone is sitting beside me, I will crush those knees in there (usually by using something to cushion them if possible). I'm fantastically anal-retentive about my personal space, so I both understand the complaints about "man-spreading" and rather fussily avoid having my own legs pressing into anyone.
3. As a male with an ordinarily sized set of genitalia, I can confirm that I can press my legs as tightly together as I can without "crushing" anything. It's not the most comfortable position in the world, but then neither is smooshing into some stranger because "mah balls just gots to be free". I will not discount the possibility that there exists a man with such extraordinarily large balls that he cannot comfortably bring his legs to shoulder width, but such a medical enormity is probably indication of a severe health problem that should be dealt with immediately.
4. As someone who rides transit every day, with very few exceptions guys are definitively the worse seat partners, to the extent that I frantically seek out a solo seat and often won't even get on the fucking train if one isn't available, choosing to wait for the next one. "Man-spreading" does occur, but usually it's lack of elbow/arm control that gets me. Yes, bus stranger, I LOVE that you've decided to read the paper and your fucking elbow is jutting into my chest so you can hold it nice and wide. Keep doing that. It's DELIGHTFUL.
5. Lest anyone think I'm just picking on guys, worst female passenger I've ever rode near was a lady who liked to take her shoes off and rub an extremely vile smelling cream on her feet for 30 minutes. I've gone to the far end of the train to avoid her.