Definitely not accidental....

In increasing degrees of privacy / decreasing amount of clothing:

This guy knows the importance of arching his back. Hmmmmm.

 The next two, I debated the order.  In this one, there is definitely dick-on-ass contact.

 

 

But in this picture, it's three in the tub, they're hotter, and each guy has the expression you would expect him to have in this situation.  The front guy is getting slammed from not one but two sets of hips.  Ouch.  The middle guy is grimacing, because a dick up the ass from that angle is not easy to handle.  Hence the value of arching your back, like the guy in the photo above.   The back guy has that glazed look of being enveloped in something snug and warm.