As far as I know, the no sex before games rule is a leftover of the discredited Victorian notion of "spermatic economy". It's not about a fine performance in the bedroom leading to a paltry one on the field. Athletes aren't supposed to cum before an event no matter what the means, because orgasms will drain a man of vital fluids and leave him listless.
From
The Lancet (free registration required):
Quote:
Nature, so it went, has endowed each male with a limited amount of the precious fluid, some of which is lost with each ejaculation. Regular (but not too frequent) evacuations are compatible with health, and many doctors, especially ardent Protestants, thought that the ideal of enforced celibacy was a Roman Catholic perversion. But too many ejaculations, and especially those of the wrong sort, could set up a vicious cycle, whereby the vital fluid was lost involuntarily, both through nocturnal emissions (a sure sign something bad was up) and a troublesome dribbling away of sperm during waking hours. The nocturnal emissions might begin with the usual erotic sensations, but as the condition took hold, these were often lost, along with restful sleep.
Local irritation of the penis, urethra, or prostate could cause spermatorrhoea, but the main culprit was masturbation, an object of concern from the 18th century for a whole range of authority figures whom the late Alex Comfort once dubbed the anxiety makers. If the 18th century medicalised masturbation, 19th-century doctors had a field day. William Acton's Functions and Disorders of the Reproductive Organs in Youth, Adult Age and Advanced Life (1857) made spermatorrhoea a central feature of disordered male sexual life. Doctors saw it everywhere, especially in their listless, nervous, and pimply patients, rendered perhaps even more anxious by the prospect of its diagnosis via the passage of a bougie in the days before rubber catheters, or its treatment by straight-jackets, genital cauterisation, perinea blisters, or constricting male chastity belts.
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Needless to say, the actual disease model of spermatorrhoea has been abandoned in mainstream medicine (although it's still around in various alternative therapies), but the idea that ejaculation results in energy loss lingers on in the wide world of sports.