Hedonist9: If you do it while he's on MDMA he's never gonna wanna do anything else. 
Also fingers (even pinky) hurt more in my experience because there isn't much padding around the bone so it's uncomfortable. More so than things which are much larger but softer and better shaped.
Innocuous: High five!
Psychetool: It's not painful. Not if it's done properly. It already stretches just fine as part of its normal function. The fun part is getting the prostate rubbed, but I have a feeling you don't know what that is. That's why it's much more fun for guys than girls.

Also fingers (even pinky) hurt more in my experience because there isn't much padding around the bone so it's uncomfortable. More so than things which are much larger but softer and better shaped.
Innocuous: High five!

Psychetool: It's not painful. Not if it's done properly. It already stretches just fine as part of its normal function. The fun part is getting the prostate rubbed, but I have a feeling you don't know what that is. That's why it's much more fun for guys than girls.