Stepmom's colonoscopy prep a la your cock - she said no surprises but tell that to the raging boner in your pants

Stepmom's googling on her phone with a very concerned look on her face. You ask what's wrong & she explains how she had a colonoscopy last week but needs to go back for a more "invasive" procedure. The doctors will stick a thick tube into her butt. She is freaking out! She's never even had butt sex before because she's so scared of anything big going IN. Stepmom says her butthole's tight & they'll need to push it in REAL DEEP. You suggest she prep herself with fingers. Stepmoms weird about doing it herself & asks for your help. She'll do the first finger, then you do the rest. No surprises either Mister! She can't believe she's about to show you her bare ass. But she's desperate & NEEDS to have this procedure tomorrow. It takes coercing but finally she pushes one finger in. For a sec she gets aroused but quickly composes herself. Now it's YOUR turn to stretch her butthole. You try to keep yourself composed as well but you can't help that raging boner in your pants...