Yoga is not Yogasana alone. It is not merely the physical. If you're interested in simply performing challenging physical postures in order to get fit whilst developing further conceits and satisfying the ever burgeoning egoic drives then do that....but don't call it yoga.
You wish to do yoga for a flatter stomach, tighter buns, do something to help with stress- these type of classes and many more in the west would lead one to believe that yoga was simply another cleverly merchandised quasi spiritual passtime with a real agenda in getting you to part with your money. That may be so, but sure even in this very limited sense this thing we call yoga can still help. But the real benefits of yoga are far deeper than this, being a way of freeing the individual from his/her past and present patterns or programming...it is a way regaining true clarity of perception.