
Well because the semen is actually staying within the anal cavity when it needs to come out. Things are meant to come out of that opening instead of going in. Just like constipation when your constipated your stomach hurt and you can also have a fever. The semen that is left behind is there for a long time can dry out and cause obstruction in the anus and bacteria. So therefore u need to wash it out thoroughly with an anal douche if you don't know how to get in deep. So in yaoi when they sleep with the semen still inside thats actually false, they will have extreme diarrhea when the awaken and feel stomach pains.

It does make sense, thank you for the info! XD I had a friend long time ago who told me how to get prepared for it, plus I'm so curious I had to investigate more about it. Fun fact? Later on that same info I used it to educate a gay friend who was just going out of the closet and was having anal for first time.
About your response to "Rianan D Halmsu", indeed yaoi manga doesn't show it all. Anal is not as easy... But they are selling us a fantasy, so as far we understand that its ok.

That's the same with females. After every time even if done with a condom, make sure to go clean yourself and pee out everything so you don't get any infections. If you've been getting uti's after doing it, basically you didn't get up to pee everything that's been put into you. All genders should clean throughly after sex otherwise this happens
( ̄∇ ̄")

Well indeed but i heard they should shower instead of pee it out, because since the flesh is more sensitive and been "rubbed" thoroughly pee could give you an yeast infection or some problems down there, that's why there's a rule to pee first and wait 30min before engaging in sexual activities :) i dont know exactly why 30min but it has something to do with vaginal discharge being able to "Clean" the Vag lol
Just because he shot a load into the uke wouldn't cause him pain, its the fact that he didnt get all the semen out after bathing which would lead to severe stomach cramps and a devils load of diarrhea. Basically a shitty stomach ache. Pun intended you have to remove all semen from the anus so that's why they should have anal douche on hand if he cant clean it properly. It takes experience.