Wow, I actually had to do a Google search for this one. Pegging? Had no idea there was a name to this. Unfortunately, I'm no expert here as I haven't had a girl shove something substantial up my ass since 1999, and even then I was so drunk and high I barely remember the aftermath.
From a physiological standpoint, however, I would venture a guess that yes, absolutely, having an object reamed up your anal cavity could put pressure on the prostate/bladder and cause you to feel like pissing. I would also venture a guess that yes, it could, in theory, damage your prostate, particularly if the object was large enough or inserted forcefully enough. No matter what misguided activists drone on about, nature did NOT intend for objects to be put up the anal cavity. Those of us who may unofficially enjoy such recreational activities do so fully understanding the risk of thwarting the natural order of things in order to act out Pornhub fantasies.
On a related note, I can't be the only person here who may (while sober) exercise caution in this arena, wary of courting a hemorrhoids flare-up?? Although I'll be the first to admit, those Preparation-H suppositories can be strangely satisfying after a few stiff triples.