I looked this up online and have seen a few remarks but nothing specific or medically based, but it seems that when I'm with another man and performing oral sex on him and he cums, I prefer not to swallow and will spit instead. However, even if a little makes it past my mouth, it will have an extreme laxative effect on me approximately one hour later. Not sure if there is any proof of causation there or if may just be sheer coincidence. Are you familiar with this?

Weak Gut

My response after the jump...

Before Debby Herbenik fielded a letter much like yours while also subbing for Dan here on SLLOTD, I wasn’t actually aware of this being a very common issue for either male or female fellators. But it may well be one of those underreported effects (and Debby also encouraged readers to email her if they’d experienced something similar, so—hoping she doesn’t mind—I’m passing on her email to you: DrDebby@MySexProfessor.com).

Debby mentioned the possible laxative effect of prostaglandins in semen, and the fact that some people may be more sensitive to these compounds than others. Just as it was for her letter-writer, that sounds like a very real possibility in your case as well. But science is about putting different hypotheses to the test, so, just for the heck of it, let’s explore a number of other theories as well.

First, as you say, correlation does not necessarily imply causation. Fortunately for you, the null hypothesis—which is that there is nothing intrinsic to semen that leads directly to you being anally expulsive after oral sex—is an eminently falsifiable one. One question to explore is whether or not this laxative effect is reliable, which is to say, does your ingestion of even modest amounts of seminal fluid always cause diarrhea, or are there ever occasions in which this doesn’t occur? This is related to the issue of generalizability. It sounds like this is a problem for you with all men rather than with just one or two ejaculators. But if it’s a spottier effect happening with some men, but not every man for whom you accidentally swallow, then you may be having an immunological reaction to that particular individual’s genetic material.

Just like our unique fingerprints, every man has a signature seminal profile (think of it as sort of fingerprint smeared with ejaculate, if that makes it easier for you—and think of that even if it doesn’t, for my amusement), and it may be the case that your own biochemistry is more compatible with that of certain men over others. The average emission consists of only 1 to 5 percent spermatozoa; the rest of the goo erupting from the urethra isn’t sperm but “seminal plasma,” a pharmacological hodgepodge of over 50 different compounds, including hormones, neurotransmitters, endorphins and immunosuppressants. Scientists are only just now beginning to understand how these active chemical factors tweak and manipulate the bodies of those who absorb them—be it vaginally, anally or through the oral mucosa. Data from heterosexual couples, at least, indicate that women who have a history of regularly performing fellatio on males with whom they later conceive are less likely to experience histocompatibility problems during pregnancy. They’re developing an early tolerance for a conceptus that will possess, of course, half that man’s foreign genome. (Of some relevance to your dilemma, perhaps, evolutionary psychologist Gordon Gallup has recently speculated that this “seminal priming” also reduces the chances of morning sickness.) For those mothers-to-be, such as lesbians, who’d prefer not to imbibe semen straight from the tap for priming purposes, sublingual sprays that are concatenations of various immune factors mixed with the donor male’s protein have been in development for years.

All of this is to say that, although histocompatibility is typically analyzed from the heterosexual lens of reproduction, there’s no obvious reason why similar mechanisms (rejection by the immune system) wouldn’t occur for semen-swapping gay men. I’m afraid the only way for you to crack this nut is to—gulp—keep swallowing and see if your condition improves as your body becomes habituated to one man’s fluids.

There are loads of other possible explanations for your semen-sensitive stomach, too. For instance, perhaps you’ve acquired something along the way like a taste aversion to semen. If, completely by chance, you happened to have a bad case of the runs after giving a guy head one day, your gastrointestinal system could have been conditioned to avoid anything it ingested just prior to this. Like eating anything else and then getting violently ill, your curious reaction could be an accident of menu choice and bad timing, rather than the intake per se. Think of wolfing down, say, an Italian sausage (no, not a double-entendre) and then having a sudden, toilet-hugging bout of the flu later that night. It doesn’t matter that it’s a virus rather than the sausage that’s really to blame; you’ll likely be steering clear of Italian sausages for some time. If you force yourself to eat one too soon, the cycle may repeat, because your gut has tagged that object as the cause of its earlier distress and, well, better safe than sorry.

Conditioned taste aversion was an adaptive mechanism, protecting our ancestors before the FDA was there to supervise the mass production of our manufactured diets. It enabled the quick, efficient, mindless identification of hazardous foods that may have made them ill—and the occasional false positive, like misjudging sausages or, perhaps, semen was preferable to getting it fatally wrong. So do you find yourself becoming nauseated at the thought of giving blowjobs generally, in anticipation of your shitty problems? If this is a lifelong issue for you (at least as lifelong as you’ve been in the business of performing oral sex) such a conditioned taste aversion can probably be ruled out, since eventually, once your body realizes it hasn’t succumbed to food poisoning and is prepared to give the food another chance, your appetite for it would return. The hour-long delay you mention also makes me suspect taste aversion isn’t the culprit.

I’ve saved the least palatable hypothesis for last, but it is, at least, food for thought. Now, how to put this in the least offensive and hideously repulsive way possible? Oh, why bother with that. Could it be that those penises that you’re prone to pleasuring with your mouth have also recently been inside the anuses of other men without their being covered by condoms? In the absence of knowing anything else about your sexual habits, your appreciation of the critical importance of using protection, or the genital hygiene of your partners, it’s within the realm of possibility that residual fecal matter containing bacteria such as E. coli is making its way down your digestive tract along with those comparatively innocuous droplets of semen.

Or, again, it could just be your sensitivity to prostaglandins.

Anyway, get your favorite (clean!) penises together and do the sticky research, WG. And while you flush out the cause, just count your lucky stars you didn’t grow up in Papua New Guinea, where for some tribes, such as the Sambia, “boys become aggressive fellators who actively pursue semen to masculinize their bodies.” You probably wouldn’t have made it very far in that society. Always on the toilet and all.