The image includes the rectum and the Large Intestine AKA colon.
That rectum is about 8 inches(12-15 cm) long and serves as the last run before stuff leaves the body.
Typically about an hour before you have to poo (roughly) fecal matter enters the rectum and meets up with the two sphincters. When you get the urge to go its because you have matter in the rectum.
Please to note that the sigmoid colon makes two 90 degree turns. And it too is roughly 8 inches long. Fecal matter can sit in there and you do not have the urge to go typically, if you are healthy (not impacted constipated, nor troubled with diarrhea).
8 inches. The average penis length is 6.5 inches. There is really no way that most men can reach the sigmoid colon.
So, how can guys take arms up to the elbow, or a traffic cone or a horse (literally a horse)? The large intestine is loosely laying in your torso. It is held in place by the abdominal wall. It can be shifted, moved, slowly about and over time you can place it permanently at a new angle.
If you are a healthy male, eating a proper diet, then you should be having a regular Bowel Movement. I have one every morning one hour after waking up. I am so regular I set clocks by it.
I eat lots of fiber (fruits and vegetables) and I keep myself hydrated (I drink lots of fluids, juice, non-caffeinated teas, water, a bit of coffee and the occasional soda).
If you know your digestive track, and if you eat properly, you shouldn't need to 'douche'. I personally do not have morning sex - ever, But then I know that i the morning I will be having my morning constitutional, thus I am an hour away from a BM.
If your stool is healthy (there is such a critter as healthy stool) then it should be firm enough to hold its shape, but fluid enough to move easily and to bend. Think soft serve Ice cream, its soft enough to bend and twirl into the cone, but hard enough to retain its shape ( bet you will never look at a chocolate soft serve ice-scream cone the same way ever again).
If its too hard, you end up ripping and scraping the sides of your colon and rectum. You do not want to ever have a man's penis up there if that happens, you are more prone to pick up nasty infections (yes even with a condom on). If too soft then you will need to douche, as the dirty water doesn't move out all the way.
Of course dirty water can be fixed by proper diet assuming you are a healthy male.