interesting thought. I think it would depend on where the curve is, and how extreme the curve is.
Here’s my guess:
the vaginal and anal canals are not rigid nor are they perfectly straight. The curve would have to be large enough to go beyond whatever the amount of flex and contour shaping the canals have.
Let’s say you have a left curving cock. If you take the far left point of the head, the left side of the origin of the curve, and create a right angle where they’d intersect. The angle of the point of the curve’s origin would have to create enough distance between the shaft and the farthest point of the head that the vaginal or anal wall couldn’t make up for it with its flexibility.
with this in mind, I’d guess that the closer to the head the curve is the less extreme the curve would have to be to be perceived as thickness. And at a certain point getting closer to the base would make any amount of curve unnoticeable because the gradient would be so low.
Allll that being thought of, it still might only be felt as being poked on the side of the wall instead of feeling like actual thickness. I don’t have a ton of experience with things going in my ass, but I have fingered my ass, I’ve used a prostrate massager (very curved) and I’ve used straight toys. My finger moving around in there doesn’t give me the same feeling of thickness as a straight toy in there, it just feels like acute pressure.