about anal sex and s*men

hellooo ^^ i am genuinely curious about this so I would be rlly glad to learn ummm basically i have read several yaoi stories wherein they clean out the s*men (inside the an*s coz they did it bareback) after seggs because the uke will get "stomach aches" if they did not...i do understand that cleaning out after a bareback is impt for hygienic purposes I just don't understand why semen will cause "stomach aches" can someone pls explain it to me?