Considering the art teacher appreciated and liked her submission, maybe assuming the teacher’s intent is misguided.

I didn't assume the teacher's intent at all. I speculated on what the comment was made in response to, based on the reaction of the person who 'received' said comment.

I mean, how likely do you think it is that an art teacher said "dial down the feminism" in response to a one-off reasonable comment about sex equality? I think there's a very good reason she didn't give more backstory, and started her anecdote at his comment, lol.

There is certainly plenty of times that men sit with legs more spread due to comfort, for sure.

I'd say it's pretty near to 100% of the time. As a male, sitting with my legs parallel feels 'unnatural' in the same way holding your hand in a fist is: I can certainly do it, but I have to deliberately do it for it to happen, that position will never happen spontaneously—it's just not the default/'neutral' position of those body parts.

There are also times that men choose to be oblivious to the comfort of those around them in certain situations ie: on public transportation, sports games, and the like where one may be close in proximity to another’s body.

This is not really gendered behavior, though, so I feel it's disingenuous to frame it as a 'men thing'. While it may not be caused by how far apart their knees are, it's arguably just as common for women to take up extra space on buses etc., by taking up either or both of the seats adjacent to them with belongings like a purse, as one example. I see no need at all to put a sex/gender filter on 'lack of consideration for the space you are taking up in a crowded location'. Deficiencies of empathy are easy to find in both sexes.

So to take a single conversation and boil it down to only 1 reason why men sit that way is pretty disingenuous.

But that is the reason the default position is different between the sexes. This:

I’ve personally had men use this “manspread” in order to press their thigh against mine in a very uncomfortable way.

assuming it was a deliberate act on his part, which you didn't state outright but implied, is in a completely different ballpark from the simple fact that men naturally sit with their knees further apart than women, pretty much without exception. And the vast majority of men who, when they notice, or it's brought to their attention, that they happen to be spread out enough that it's taking up another seat, will readily adjust themselves to accommodate the other person, because they're obviously not acting maliciously.