Evolutionary processes have shaped the human penis to scoop previously deposited semen out of the vagina during intercourse in order to prevent impregnation by another male. The cessation of thrusting during the male orgasm prevents this scooping of semen from occurring. The recovery time for men in between orgasms is thought to be a mechanism for preventing the male from removing his previous ejaculate from his partner's vagina and thus potentially lowering chance of impregnation.
Source: Scientific American
Image source: Wikipedia