What is a foundation for the alleged moral obligation to intervene under the circumstances in this case (child rape or sodomy)?
Enlightened self interest.
The person who doesn't intervene will then suffer years of regret and shame, or they will have to deaden their soul to avoid the regret and shame. The person who does intervene will experience empowerment and pride. Not a complicated calculation.
The obligation is imposed by society, out of it's self interest. None of us want to live in a world where children and other innocents do not occupy a special status. It's in our interest to put social pressure upon those in a position to intervene.
I grew up in a football culture, played the game throughout my youth, and thus am familiar with the lessons in manhood the game tries to teach. The Penn State story clearly illustrates there is more to manhood than big muscles and public success. Hopefully this sad story will be used to better teach the next generation of young men about what manhood really is.
Finally, I feel we should be wary about over analyzing such topics. I know that philosophers are rightfully averse to taking anything on faith, but there really is a valid role for faith in some key aspects of human life. I don't mean religious faith, but moral clarity.
The appropriate response to child rape is not to sit back in the academic ivory tower arm chair, puff wisely upon our pipes, and use such events for our intellectual entertainment.
The appropriate response to child rape is instinctive gut level decisive action.
The Penn State coaches and University officials tried to use their clever heads to react to this situation, and it got them in a boat load of trouble.
If they had used their hearts instead, if they had acted instinctively with decisive unblinking unambiguous moral clarity, they'd still have their jobs, and a number of kids would still have their innocence.
Reflection, analysis and intellectual entertainment is the not solution to every problem.