My experience is that pegging does not effect the prostrate as it goes much further.

in terms of the diameter and the length of the inserted object, the force that is used etc but in general:
pegging reaches ans stimulates the prostate and can easily produce a climax.  It is often a complex invasive/unpleasant/very exciting and pleasant sensation.
The prostate is so affected that pegging, or other prostate message, often raises the PSA (as does the simple act of climaxing.

I am pretty sure it's the same thing that happens to the clit when a woman gets fucked.

The cock (or dildo) may or may not brush against it, stimulating it.  Depends on angle, position, individual anatomy, etc.

If you are new to pegging, it is important to know that harm CAN be done- go slow, use lube, stimulate stimulate stimulate before hand, fingers knuckles whatnot, reacharounds are pretty great too your first time, and find someone who knows what their doing.

   You don't want a dirty strap shoved up your ass w/o any premeasures, and you don't want to look in your underwear the next day and it looks like you just ragged.

   Strap-on play can be very pleasurable for both parties, just make sure both parties are completely aware of the posibilities, negative &positive. Find someone who really knows how to fuck an ass, and can prepare your boy pussy for penetration. Get it used to different sizes, see how much you can take.

   And to answer your original question, it does & can stimulate the prostate, (especially if the provider is familiar w/more positions than just "bend over boyfriend") & they even make special straps to stimulate your prostate even better so you can bust a real nut.

   Play hard & be safe.

-- Modified on 12/23/2016 1:21:07 PM

The prostate is a walnut size oran that sits just in front of your rectum and just behind your bladder. It is easy to reach.  I have short fingers but can general feel most of the exposed surface of most men's prostates when i do a rectal examination.  
Massaging the prostate is not a bad thing and in some conditions is therapeutic.
It is hard to inserts things in your rectum (finger, a large stool passing out of your rectum, a strap on, etc) without massaging or at least pressing on the prostate.  If done right, most men find this pleasurable in the right setting. (Few men enjoy a rectal examination but most enjoy a well done prosate message and many men or trans genders find anal sex very enjoyable. And many find strap on and other forms of pegging enjoyable.) The use of rectal beads inserted slowly and pulled out at the right moment can make climax magic.
That being said this can be painful and do harm if not done right. The harms is usually not to the prostate but to the rectal sphincter. Like with anything inserted, careful sterility is important, as well as the right size and the right technique.