For me, I've found that anything worn in shorts pockets interferes with the cycling effort, except for the most casual cycling. Even then, it becomes annoying after 30 minutes.
Most cycling shirts and jackets are very lightweight material, and even a P32 will sag in it. Putting the pistol in the rear pocket is not very accessible to you, and quite available to someone attacking you. You also run the risk of losing it if it falls out.
After many go-rounds with this, my best two choices are either carrying it on the bike (small frame- or handlebar-mounted pack), or in a small waist pack. On a touring bike the handlebar mount works nicely, otherwise the small frame pack will do. Both are secure and accessible if you are attacked on the bike. When you dismount, you will have to be discreet to retrieve it and pop it inside your waistband.
The small waist pack is a good choice, especially if you are dismounting frequently. You can keep some small change and ID in there as well. On the open road at speed you can slide the pack around on your lower back, freeing your waistline for aero positions. For around town or casual biking, the front carry is unobtrusive and readily accessible.
BTW... the P-32 is a great choice of this activity, where every ounce of weight increases your effort. The light recoil allows to you shoot one-handed accurately if necessary while controlling the bike. If the 10-shot magazine fits your carry options, it's a good choice in the pistol.
YMMV