I think for most men it's the anus. If you read U.S. military discussion forums talking the physical that new recruits have to go through at MEPS (military entrance processing center), you'll notice that applicants are generally fine with having to strip naked and get a hernia exam but they really hate having to bend over and spread their butt cheeks to expose their anus:
https://www.reddit.com/r/newtothenavy/comments/m14pv9/spreading_butt/
https://www.reddit.com/r/army/comments/1o0sni/why_the_hell_did_they_look_at_our_buttholes/
https://www.reddit.com/r/AirForceRecruits/comments/s8wfgv/do_they_have_to_check_ur_butthole_at_meps/
https://www.reddit.com/r/newtothenavy/comments/dyw9nj/does_anything_go_inside_my_anus_while_taking_the/
https://www.reddit.com/r/Military/comments/61hz7a/eh_more_or_less/
https://www.reddit.com/r/AirForce/comments/k6lppl/meps_was_definitely_a_good_time/
https://www.reddit.com/r/AirForce/comments/lqyojr/bend_over_and_spread_your_cheeks/
https://www.reddit.com/r/navy/comments/hi5rdz/spread_em/
https://www.reddit.com/r/AirForce/comments/gsg92l/spread_em/
https://www.reddit.com/r/Military/comments/9bvx9e/do_you_ever_miss_it/
https://www.reddit.com/r/AirForce/comments/iioib4/its_literally_the_first_thing_we_do/
https://www.reddit.com/r/AirForce/comments/he1ka7/reveal_your_rectum/
https://www.reddit.com/r/AirForce/comments/i4744r/go_ahead_and_cough_for_me/
https://www.reddit.com/r/USMC/comments/x93tz8/remembering_meps_with_friends/
https://www.reddit.com/r/army/comments/mx0era/why_did_they_finger_my_anus_for_sf/
https://www.reddit.com/r/navy/comments/f82hp0/dont_say_anything_to_the_doctor_at_meps_every/
https://www.reddit.com/r/AirForce/comments/lks862/oldie_but_a_goodie/
https://www.reddit.com/r/Military/comments/9jhwei/is_it_normal_for_an_army_doctor_to_ask_to_see/
https://www.reddit.com/r/Militaryfaq/comments/euuu5x/military_anal_exam/
https://www.reddit.com/r/AirForceRecruits/comments/u1c1qn/serious_should_i_shave_my_ass_for_meps/
https://www.reddit.com/r/AirForce/comments/pbpcav/meps_flashbacks/
https://www.reddit.com/r/AirForce/comments/ovr5j6/go_ahead_and_cough_for_me/
https://www.reddit.com/r/USMC/comments/u5e0g9/did_you_get_your_butthole_fingered_at_mpes/
https://www.reddit.com/r/AirForceRecruits/comments/r1cre9/going_to_meps_next_week_what_was_your_experience/
https://www.reddit.com/r/nationalguard/comments/jcunp5/meps/