Now, as a man, your opinion on, say, anything to do with testicles (this is just an example; there are plenty of things men know firsthand that have little to do with testicles) is inherently valid, because you have them, so I'm not suggesting for a second that nothing men say is inherently valid.
So take the average straight, white, middle-class, able cisman. Going through one's life without ever having one's opinion questioned on the basis of one's sexual orientation, race, class, physical ability, or gender translates to behaving as though one's opinion will never be questioned -- for any reason at all. I don't know why, but somehow it seems it does. So it doesn't really matter whether or not you meant to mansplain, or whether you're deliberately being sexist, or if that's just how men are, because you don't need to have intention to mansplain.
A lot of mansplainers have intention; a lot don't. The fact remains that men exist who, whether they acknowledge it or not, have underlying hostilities toward the idea that a woman might know more than they do, or be able to do something better than they can.
Case in point: Ex-boyfriend.
- It drove him nuts that I was more academic than he was. There's no clear reason for this, as he'd never considered himself an intellectual before I showed up, so I can only assume he just didn't want his girlfriend to be smarter than he was.
- It drove me nuts that he constantly beat me at Scrabble. This was because I was clearly more word-minded than he was, and I played Word Scrabble, whereas he played Points Scrabble, and kept kicking my ass.
Incidentally, "men are just dicks" is the most mansplaining mansplanation for mansplaining one could possible think of. It's circular, and if you don't see that, I really don't know how to explain it to you.
In this particular thread, the guys who showed up to complain about "reverse sexism" and "I'm not a mansplainer, it's just the way I am" displayed absolutely classic mansplaining behavior -- most strikingly, after their initial complaint, when the women in the thread refused to engage, merely laughing at them. This, of course, just made them mad, because how dare these shrieking harpies not take them seriously?!?
And that, in a nutshell, is mansplaining. "I assume you will listen to me and take me seriously, no matter what I'm talking about, because I'm used to that, because I am a man."