Back in the wild hippie days, I helped Margo St. James found a hookers organization called Coyote. I gave her an office and some foundation money to get started as well as abundant operational advice.
This is by way of introducing a subject my readers will never have encountered anywhere else. I got to meet many hookers (street working prostitutes) as well as call girls and others in the trade. I met them in unthreatening, none business social situations.
One old tale that was widely believed was that sex with many men would keep a woman from getting pregnant; that men’s sperm would fight each other’s.
Years later, when I was involved with the research done for the Institute for the Advance Study of Human Sexuality I learned that the old tale from hookers was probably right.
There are three kinds of human spermatozoa. One aims just to impregnate the ovum. One aims to attack spermatozoa from other males. The third is intended to create a more acidic vaginal environment to thwart other males' sperm.
The ratio of these three types of sperm varies with the occasion; it depends on frequency of ejaculate, time of separation from the female and other situational contexts.
The other related sperm phenomenon that was passed on from old hooker tales and was confirmed with recent scientific methods is that sperm success is sometimes related to female orgasm. In careful studies it has been found that after a female organism the cervex is likely to dilate and allow entrance to more sperm. In some instances the uterus also displays a pulsating movement that moves the ejaculate toward the ovum.
If you are looking for the original research on these issues add the term ‘academic’ to your google search and be prepared for paywalls around every article.
Here is a good summary of the research.