In the marketing materials for vasectomy, you frequently read assurances that vasectomy will not affect sex in any way. One common way to reassure men is to tell them that sperm only constitute a very small percentage of the ejaculate, and that you cannot therefore detect any difference. The percentages quoted seem to vary quite a bit. I don’t have a rigorous survey of public statement for this, but my recollection is that people state numbers in the 1% to 5% range.
I found a study recently that tried to quantify this back in 1979:
Small sample size here, but you can see that the decrease in volume is 3% which agrees with what doctors are eager to claim. Indeed, this appears to be the claim the experimenters were investigating, as they conclude that:
These findings support the contention that the volume of semen originating in the testis, efferent ducts and epididymis is very small compared to the volume originating in the prostate and seminal vesicles
But what about that viscosity measurement? That’s a 31% drop. Seem a little shocking that a 3% drop in volume is creating such a large drop in viscosity? It turns out that it doesn’t take very much of change in composition to radically change the viscosity. For example, if you took the gelatin out of a cup of jello, the weight would only change by 2%, but the viscosity would drop by 99.9% I’m willing to bet good money that ejaculating jello would feel pretty different than ejaculating water, even if the volume was exactly the same.
Unsurprisingly, you do not hear doctors cheerfully reassure men that vasectomy will only decrease the viscosity of their semen by 31%. It’s hard to see how a 3% change in volume could affect the sensation of ejaculating, but one might reasonably wonder whether a 31% change in viscosity might result in a sensation that is less like ejaculating and more like urinating. In my opinion, the set of facts that men are given when deciding whether or not to get a vasectomy are not filtered in an unbiased way. Rather, there is the “correct conclusion” that vasectomy is safe and desirable, and facts that threaten this narrative have a hard time getting themselves transmitted to men who are considering whether or not to get a vasectomy.
Some men refer to semen after vasectomy as reminiscent of skim-milk, but when it comes to viscosity specifically, a more reliable comparison is the difference between cold milk and room-temperature milk, which is about 34% less viscous: (Link)
Just one study, so take it for what it’s worth. Does anyone have more science on this?