Ben Grimm got the shit end of the Fantastic stick when it comes to powers. Of a four-person team, he's the only one who came out looking like a turd rolled in walnuts, while the rest of his team can blend into polite society any time they please. Sure, he's super strong and nearly invulnerable, but what good is that if every time you try to jack off you just make a handful of gravel?
It stands to reason that the cosmic radiation that made The Thing's body into an old country road probably didn't tactfully or thoughtfully avoid his junk. There's no stony flap in his drawers that he can push aside to get out Lil' Grimmy and please the ladies, or even pee properly. The internal logic of the Fantastic Four dictates Ben Grimm has a stone dick. And hey, that's cool; archaeological evidence indicates our ancestors used stone dildos. There's probably a neighborhood out there in time and space where Ben will fit right in.
If you know what I mean ...