The pushup, situp thing was a general fitness question, and I suppose general fitness can be done elsewhere at a different time. However, people who do other exercise (running, weights, swimming ... whatever) MUST warm up and stretch their tight muscles and joints before class.
When I hear people talk about five minutes of warm ups/stretching, it makes me nervous. OK if you do your own thing before class, but only five minutes total? What do you stretch, just your legs? Just your wrists and shoulders?