Regardless of what type of saddle I'm riding, when I post it seems my knees have the most contact, followed by my calves. When riding my forward seat in two point or an exercise saddle, my thighs have little contact with the saddle.
I ranch in addition to other things, and every single rancher/cowboy I know posts at times. If you are trotting a few miles, you want a good ground covering, working trot. Posting, done well, gives your horse a change. Sure, we sit the trot at times, too. As far as I'm concerned you never had to explain yourself for posting in a western saddle.
BTW, a horse with a "big trot" is valued out here.