What he was "up to" was trying to save your life.

What if you had prostate cancer that had spread to your arm, and
that's why your arm was hurting? While you didn't, as usual, bother
telling us anything fundamental about you like, oh, say, your age
(like most idiots who post on sci.med with stupid questions like
yours), you certainly didn't hesitate to tell us that your response,
which you admit is based solely on your own ignorance, to a physician
who is trying to be thorough (when, by contrast, most complaints on
here are about physicians who are cursory and brief) is to spew
profanities at him and accuse him of being a pervert (as if doctors
enjoy digital rectal exams any more than you do).

http://www.mdanderson.org/topics/paincontrol/display.cfm?id=FB5B544D-3357-11D5-8
11200508B603A14&method=displayfull

What if he checked only your arm and prescribed pain medicines, and
you eventually became gravely ill and were later diagnosed with
prostate cancer by another doctor? You'd be on here complaining that
the first doctor didn't do a rectal exam when you came in with arm
pain, and I'm sure you'd be looking for a malpractice attorney!

PF