Step-son, I know you've been sneaking sniffs of my socks, and peeks at my feet, so, now that you're 18, I want to give you a graduation present... You can jerk off to your step-mommy's feet! BUT! THere's a catch... After you're done you have to put on this chastity device. I want you paying attention in college so you can graduate and get a good job to take care of me.
Every school break you can come home and jerk to my feet again without the device on.
Deal? Good. Now cum for my wrinkled soles.