So, I'm done with my work and wrapping things up for the night, so I can spend a minute or two with this...

Onanism is a term that has come to mean "masturbation" after Onan in the Bible who had sex with his brother's wife, but did not come inside her because he did not want to get her pregnant. He wasted his seed, and God killed him for it. Wasting semen = masturbation. But, more correctly, what Onan did was practice the ancient form of birth control called Coitus Interruptus.

There is a huge myth that the only reason why coitus interruptus fails as a method of birth control is when the man loses his self control close to orgasm and ejaculates inside the woman. Sure, that is the most common reason why it fails, but not the only reason.

There are viable sperm present in "pre-cum" (called Cowper's Fluid) that is excreted well in advance of actual orgasm. Additionally, some actual ejaculate may occasionally begin to be expelled just a few seconds before the feeling of orgasm in some men, sometimes. Additionally, after ejaculating outside a woman, some men feel it is safe to re-insert their penis inside a woman or rub against the vulva and a significant amount of sperm remains in the urethra that can be expelled after orgasm is completed and even sperm around the outside of the vagina can migrate inside the vagina through minimal amounts of manual manipulation or even the act of putting on tight fitting underwear or pants. Finally, sperm continue to remain viable inside the man's urethra for hours (maybe even days) so that if he has recently masturbated or had sex with a woman and then some time later has vaginal sex with a woman and still practices coitus interruptus he can deposit a relatively large amount of sperm as the "pre-cum" is deposited in the vagina during the subsequent acts. The best defense against this is to urinate several times between each sex act.

The current position paper by the American College of Obstetrics and Gynecology regarding the pitfalls of coitus interruptus as a means of preventing pregnancy can give you much more detail; however, this is what the ad was getting at. Coitus interruptus (e.g. "a pop shot on your tummy") has a failure rate of up to 25%.