Portraits with female subjects learning against something are fairly common, and can be done quite well (or not well) depending upon how the subject poses and the camera position.
In almost every case of portrait photography, the photographer will want to flatter the subject and well as draw the viewer into the image. Looking at the example image above, if the model were flat against the column the image would be far less interesting. In some cases, the subject will place her back flat against an object with stiff legs. If we examine what makes this image interesting is that the model:
By choosing a camera position about 45 degrees from the front of the model, the photographer has been able to capture the the absolute lines in the body as well as bring some depth into the images with the additional columns.
Despite the semi-relaxed look of the model, she is maintaining a good posture and she is using at least two contact points (three if you count the camera side arm) to define her form. What seems almost as a casual shot, turns out to be a planned image to draw the viewer into the image. The photographer of course used a selective depth of field to keep his model sharp while columns became less sharp.
Just some ideas to ponder the next time you plan a 'leaning portrait shot'