I wear these exclusively for exercise, especially on the elliptical trainer and treadmill.  I chose them to prevent chafing between my legs, and the length of these is just fine in that regard.  No chafing issues at all.  While they fit a little loosely on me for their size, they are quite comfortable, and they stretch easily without binding or riding down too far in the back when using a weight machine.  They wash well and have kept their original size and shape--I just have to be careful to dry on low heat or hang to dry, because of the nature of the fabric.  For most of my life, I have stuck with cotton underwear, because they are comfy and absorb sweat well.  I have stayed away from synthetics in years past because they never used to absorb sweat, but the modern dry-fit polyester fabric does the job for me, and I'm a happy camper with these for exercise.