I have absolutely zero issue with taking a similar pose with the same spinal curvature. It's not particularly comfortable but it's really not physically impossible, and I'm not at all a flexible person... In fact I had spinal surgery that drastically reduced my flexibility.
The issue with the lineart is somewhat ridiculous. Sorry, I have no other words left for that part. However, the problem is more with the leg placement (especially her lower kneecap looks like it's the end of the end) and the slightly derp face. Spine twist isn't really an issue here. it's a fairly simple post.
Dogs are not human. And even not considering that, you can still clearly see how her pelvis would have to be shifted of a good distance to the left for this pose to work.
Her torso is completely facing the viewer with no suggestion of turning taking place.
Your example don't even compare, because all save one show a minimum of perspective that don't make the lower and upper body completely disconnected - and the only example that does not is evident chibi artstyle that has to be judged differently.
Finally, it doesn't help that the legs contribute to the confusion by not being clearly discernible in the background. It's an overall pretty confusing picture.
you can still clearly see how her pelvis would have to be shifted of a good distance to the left for this pose to work.
Her torso is completely facing the viewer with no suggestion of turning taking place.
Based on what we can see of the legs and panties (that green/teal fabric visible under her right arm pit), I do not think her pelvis is out of alignment of human possibility.
Look at her shoulders and collarbone. Her right side is higher than her left, meaning that the torso is angled to accommodate for what is taking place behind.
Unfortunately we can't see what's going on behind her due to most of it being hidden from our perspective, but I can imagine something like this posture from the waist down if we were looking from the side.
Your example don't even compare, because all save one show a minimum of perspective that don't make the lower and upper body completely disconnected - and the only example that does not is evident chibi artstyle that has to be judged differently.
Finally, it doesn't help that the legs contribute to the confusion by not being clearly discernible in the background. It's an overall pretty confusing picture.
All those examples are pretty much the same pose where we can't see the waist/pelvis due to perspective. Their legs peak out from the side at comparable angles and displacements as well. Either all them have the same problem or none of them have a problem.
As for the legs being the way they are, please refer to this again as a example of what the legs are doing if we were to observe this pose from another angle.
In post #1666866 for example, the woman's clearly laying prone and pushing her shoulders up by resting on her elbows. Even at her shoulders, she appears to be leaning about 30 degrees off of horizontal towards the viewer. (In post #1688286 and post #3049001 they are nearly completely horizontal.) Here, she seems to be at a 90-degree perpendicular angle, especially if you're looking at her shoulders/clavicles, which makes the apparent bending of the torso much more extreme. It's also hard to tell the angle the chest around the breasts is at. Yeah, you can see that and say it's a really baggy shirt that's hanging down past the breasts even though it's obviously tight enough to be curving back under the breasts, but my first glance at this picture didn't suggest that to me, and it looks more like she's vertical until very sharply bending around sternum-level. That it's a sort of optical illusion where what the pose is supposed to be looks different depending on where on the body you are looking, then there's a problem.
I don't particularly care about this one either way - it's FAR from Ryo Agawa's worst anatomy atrocity - but I can see why people would flag this one.