Is it okay to get an erection at a prostate exam?

yes it is

Are you the doctor?

it would amuse the dr

No, it's not OK, you're crossing a line with the Dr. The exam is usually a pretty quick one, It certainly does not go on long enough for the Dr. to stimulate you into getting an erection, he/she knows this and also knows that if you get an erection it's because you have some unprofessional fantasy going on inside your head.

Well, I just had a prostate exam with my female Dr. She seemed to leave her finger(s) inside me for quite awhile. I felt my penis becoming erect, and knew she would be expecting this, so I let it happen. She said everything felt very normal, and as I turned toward her still nude, my partial erection (about half to 3/4 erect) was plainly visible. She said very professionally that males do get erections during prostate exams sometimes. We're talking about purely heterosexual males, not gay men. I'm hetero, but have had this happen with a male Dr. too.

It is if you have a lady doctor.

Only if the doctor is a female.