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Uhh… what I learned was that the muscles in your pelvic contracts during post-orgasm, which can lead to cramps due to its speed. But don’t quote me on that lmao.
That or when a yaoi top’s penis enters the bottom’s anal canal, their cum is so explosive it affects the bottom’s gastrointestinal system.
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I found this answer on Quora from another user: Seminal fluid has prostaglandins which stimulate muscle contraction and spermatozoa have acrosomes which release their irritating contents when the sperm cell dies, or thinks it's contacted an ovum. This may be why diarrhea, cramps, or at least urgency or loose stools are not uncommon an hour or two later. However, many people report they have none of these symptoms and that the semen either disappears or comes out much as it went in.
Personal note: I never had this experience, but I indeed heard a lot about the diarrhea and people using anal sex as a solution for constipation (lol) so I guess the pain yaoi usually refers in those cases is the urge to evacuate (but this is just a guess, I'm neither a scientist nor had cum in my ass to say for sure) ╮( ̄▽ ̄)╭
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Also a personal note, I mean experience: I think it sometimes differs on your stomach's condition probably that day lol. There were times when I had stomachache after there were times that there's none. Maybe it's how much was let out inside that's why it upset my stomach or how intense the thrusting was, but one this is for sure, it doesnt stay inside there it just normally flows out after.
why do they always say “you’ll get a stomachache if I cum inside without a condom” someone please explain so I can delete this comment and act like I never asked