Yes, I do indeed believe that means you do not understand those "questions". As such, your insistence that major issues deeply dividing society are solely one-sided, and establishing policies based on the one-sided view, leads to what I shall colloquially refer to as "civil unrest". Don't dismiss the guideline simply because it doesn't lead you to where you want to go.
> I can't make a strong argument for the Earth being flat, does that mean I don't understand the question well enough?
Yes.
Have you never used a flat map for navigation? Did the map deviate from what you observed on the earth by enough to notice? The curvature of the earth is very small, so it is trivial to come up with a world-view in which it would be "obvious" that the earth is flat.
Lest one dismiss the notion that anyone would willingly adhere to a "flat Earth world view", remember that most people are baffled by airliner flight paths as drawn on flat world maps.
"In the early days of civilization, the general feeling was that the earth was flat. This was not because people were stupid, or because they were intent on believing silly things. They felt it was flat on the basis of sound evidence. It was not just a matter of "That's how it looks," because the earth does not look flat. It looks chaotically bumpy, with hills, valleys, ravines, cliffs, and so on.
Of course there are plains where, over limited areas, the earth's surface does look fairly flat. One of those plains is in the Tigris-Euphrates area, where the first historical civilization (one with writing) developed, that of the Sumerians.
Perhaps it was the appearance of the plain that persuaded the clever Sumerians to accept the generalization that the earth was flat; that if you somehow evened out all the elevations and depressions, you would be left with flatness. Contributing to the notion may have been the fact that stretches of water (ponds and lakes) looked pretty flat on quiet days.
Another way of looking at it is to ask what is the "curvature" of the earth's surface Over a considerable length, how much does the surface deviate (on the average) from perfect flatness. The flat-earth theory would make it seem that the surface doesn't deviate from flatness at all, that its curvature is 0 to the mile.
Nowadays, of course, we are taught that the flat-earth theory is wrong; that it is all wrong, terribly wrong, absolutely. But it isn't. The curvature of the earth is nearly 0 per mile, so that although the flat-earth theory is wrong, it happens to be nearly right. That's why the theory lasted so long."
In many ways, it's really just a matter of measurement power. An illuminating illustration of this is posed by how far away do you need to be from another person to see them disappear over the horizon [0]. Turns out it's like 6 miles. I have terrible eyesight, so without telescopic optics, I'd never have been able to measure this.
Rather, I'd have to rely on local measurements. And that'd nail me too: the Earth only curves about 12cm / km. So if I could only resolve a local rise-over-run of 1/1000, I wouldn't be able to fail the null hypothesis that the Earth is flat. (But if I could manage an order of magnitude better, I could!) And given that hills and such are all kinds of lumpy, and large bodies of water are rarely still, getting even that level of resolution without advanced optics would be difficult. (Though if you can be sure you've got a straight enough stick...)
So I think it really comes down to how well you can prove or measure anything. Once we had telescopes, there really wasn't too much confusion about the spherical nature of the planet. (And people had suspected for a very long time the earth was - at least in some way - round. Eclipses give that away a bit.) But the details really give us the resolving power to be sure. That and it helps to get away from local measurements - get up really high, and it becomes easier to tell (and IIRC some early experiments measuring the size of the Earth took advantage of really large height differences).
After all, Newton was right, too. But add a few extra zeroes to the solutions, and we start seeing some deviation from our relative measurements...
Of late I've been taken by the concept that someone who grew up on the far side of the Moon, and never travelled very far, would - due to the Moon's rotation period equalling its revolution period - be completely unaware of the existence of the Earth. They'd see the Sun rise and set in a 175 hour cycle, along with the rest of the Universe. The notion that a brightly reflective body, significantly larger than their own spheroid and covered with billions of intelligent (?) beings, existed a mere 238,900 miles away would be absolutely preposterous ... at least until said resident travelled far enough to peek around the horizon and see a most mind-blowing sight.
I mention that to set premise that one can be remarkably unaware of a plain truth just around the corner. The strong argument for Earth being flat is little different from the strong argument for Earth perceived as flat. The flaw obviously is the objective difference between fact & perception.
I go thru trouble of writing this to note that while you're pointing fingers at the difference between being and perceiving, you are yourself holding the mistaken notion that Earth is a lumpy sphere, when Earth is, in objective reality, a very long and slightly bent 4-dimensional _spring_ shape, which we see just a 3-d cross-section of which looks spherical to us lower-dimensional beings.
While making a strong argument, be humble - your perception may be objectively wrong, misguided, or incomplete as well.
The arguments are not meaningfully different. For each individual, their perception is indistinguishable (to them) from reality. It could be trivially changed:
"I use a flat map to navigate. I perceive no difference between the map and reality. Therefore reality is like the map."
For things that are very strongly one-sided you almost certainly aren't going to make an argument that convinces yourself, but if it's not at least as good as the arguments used by people that disagree with you, you are doing yourself an intellectual disservice.
I can't come up with a single coherent good argument against gay marriage, does that mean I don't understand the question?
I think your rule is very useful as a guide line, but please don't make it a rule. There really are policies and ideas that are one sided.