I think that this is a complex question. On the one hand, the prospect of being examined by a female for a straight male likely is laden with sexual preoccupation. The very fact that one is surrendering to the female and exposed in ways that reveal all is a powerful source of arousal. It doesn't take much for the penis to react to touch and touch there will be. It's only a matter of when and whether one's briefs are off or merely lowered. Surely one will avoid eye contact as she grasps our sack. Genital and likely rectal examination has to heighten one's sensitivity to respond. Responding to her commands is absolute. One doesn't want to appear sheepish. Secretly, one wants the penis to look bold and not shriveled. The fear of appearing inferior; not measuring up is huge. Alas, there is no guarantee when it is being held between her fingers. The gentle rotation of each testicle reminds you that you are at her mercy. At this point, tension merely builds. So for most of us, the embarrassment would be huge along with the inevitable spontaneous erection. Then comes the directive for your DRE to which you obey but may fantasize as her finger twists fully up your passage.
As for a male to male exam, the tables are turned. Throughout all of this, the sensitivity to touch and reaction of the penis and anus is like no other moment.
Here, the focus may be more on what is to happen. The sequence of events sets in motion a very strong psychological process, if you will, that has us wondering how much of me will he see and touch. The time spent waiting for the doctor leads to tension and likely erection activity. For some, the days preceding the exam is met with ongoing masturbation and reliving previous physicals. Once there, one is likely to ruminate about undressing as if being strategically stripped or required to show all and to do so promptly. The sudden awareness of “How will I measure up,” happens fast, and then the evidence is revealed as one lowers his underpants. The doing of this is so personal. The only question is whether to be bold and lower your briefs while facing the examiner or turn as if to shield yourself. Of course, the doctor can trump all by saying, “Toss your briefs on the chair and stand here.” There could be other variations like having to raise one’s hips and be told to lower or more importantly, remove one’s underpants. Now you are naked and there is no hiding.
I suspect for the male to male exam, there may be less thought about using a modesty sheet since, “Heck, we are all males here.”
For some, this moment is met by a gradual erection such that it engenders pride when the penis assumes the “L” position. The doctor has to navigate this with skill and gentleness. The inevitable testicular exam heightens awareness and if erect, only adds to the possibility of producing some precum. The obligatory hernia exam and the necessity to cough while his finger pushes up the inguinal canal has to garner one’s attention. It is only moments away from the snap of fresh gloves, the appearance of KY, and the directive to either turn onto one’s side or bend over the exam table. If your doc is very discipline, you could have two rectal exams given, one in each position.
Regardless of the sex of the examiner, the exam when done completely is a full on journey into becoming submissive and allowing for one’s private parts to become the focus on attention. How you react is a product of past experiences as well as fantasy thoughts and the ability to allow yourself to be “present in the moment” when it’s “Your Turn.”