If “Josh” were to have sex with “Kate” who recently had sex with “Mike,” in the process of thrusting his penis back and forth in her vagina, some of Mike’s semen would be forced under Josh’s frenulum, collect behind his coronal ridge, and displaced from the area proximate to the cervix. After Josh ejaculates and substitutes his semen for that of the other male, as he withdraws from the vagina some of Mike’s semen will still be present on the shaft of his penis and behind his coronal ridge. As his erection subsides the glans will withdraw under the foreskin, raising the possibility that some of Mike’s semen could be captured underneath the foreskin and behind the coronal ridge in the process. Were Josh to then have sex with “Amy” several hours later, it is possible that some of the displaced semen from Mike would still be present under his foreskin and thus may be unwittingly transmitted to Amy who, in turn, could then be impregnated by Mike’s sperm."

— Secrets of the Phallus: Why Is the Penis Shaped Like That?: Scientific American