Crossing the legs, though often comfortable, doesn’t serve us best. A pelvis that lives in a state of balance will allow for a spine that can be balanced successfully as well. The pelvis is three bones—the two hip bones with the triangular sacrum wedged in between. If the hips are level to the earth and in the same plane at the front and back of the body the legs will be able to sit evenly into their sockets and the spine can stack straight up to the sky.
A neutral or balanced pelvis is an ideal worth searching for. When I sit in my desk chair with my feet flat on the floor I am hoping to have my weight placed evenly across both sitting bones from side to side, and the front and back of the pelvis should be organized in such a way as to bring the tiniest of arches to the lower spine. These two markers (weight across the sitting bones and an arch in the lumbar spine) denote a balanced pelvis. Any time you are crossing your legs you are throwing off any chance to maintain this balance.
As I type this my right ankle is crossed over my left knee. Such is my lot in life. If I lay down on a couch, my right knee bends and my foot trails up the inside of the leg in search of freedom for a tighter hip socket. My hips are loose in general but the right hip is slightly tighter than the left and always looking for release. With the right ankle across the left knee the leg bone is being rotated out of its socket and the weight of the pelvis is falling to the right.
Sitting correctly on a chair means sitting with my feet flat of the floor and the head of my femur bones—where the leg meets the pelvis—screwed into the hip socket in a solid manner. But if I live with an habitually tucked pelvis and legs that leaned forward of the pelvis, sitting with the legs situated correctly with feel uncomfortable.
Unfortunately we tend to alleviate this discomfort, rather than fixing it, by crossing the legs. Crossing one of the legs lessens that discomfort by taking the pelvis out of a neutral position and throwing off its position to be more like the imbalanced pelvis you usually live with.
It would be best to sit at your desk, on the train, at the dinner table with the feet flat to the floor and the lower back in a natural arch. If that is not going to happen. try avoid crossing only one leg. Do your best to cross from both sides.