Couple of months ago I was in Bali and I saw local women weaving fabrics. They would sit for hours with their legs straight on the ground and a perfectly straight back, like a letter L; no back support of any kind (try it!). Bad back? That's western problem. We sit in our "comfortable" chairs with "lumbar support" and slowly decay.
When I started doing deadlifts I found the posture quite strange and the motion felt very unnatural. Too many things too keep track of. But my trainer would point out all my mistakes and I'd improve, bit by bit. After few months the results are great.
Standing is in the same bucket. It is a physical activity. One shouldn't assume it is something that humans are just supposed to be able to do right without practice. Most certainly not after years of bad habits. If you don't have the proper physique it's going to hurt. If your posture is bad it is going to hurt. So figure out what are you doing wrong and fix it!