There's one spot on your bike that many cyclists overlook as a potential place that can rub and damage your bike shorts. That's the velcro closure on your bike seat bag, specifically the one that secures your seat bag to your seat post.
Many bikers do not realize how important it is that the velcro tabs be as tightly wrapped around your seatpost as possible. This should go beyond even what is needed to hold the seat bag on securely. Wrapping the straps even tighter allows the two pieces of velcro attachment material to overlap as completely as possible, covering the strap with the rough side of the velcro more completely with the smooth side. Not only does that make the bag hold more securely, but more of the rough side that is covered, the less rough material is exposed to then rub against your bike shorts on your inner thigh, almost down near your backside. This is where your legs will often brush against your bike as they go up and down like pistons during your pedaling motion.