While human copulation can take many forms, we’ll focus here on the basics of procreative sex, whereby male ejaculate is used to fertilize a female egg. Human copulation typically begins with arousal. For females, this is accompanied by the production and secretion of vaginal fluids that can signal arousal and enable easier penetration. For males, arousal is accompanied by an erection (or “hard on,” “boner,” or any number of other euphemisms), which is itself the result of rapid movement of blood into the spongy tissues of the penis. This blood causes the spongy area to swell, and makes the entire penis stiff.
An erect penis can penetrate a lubricated vagina. Humans (and many other animals) typically follow penetration by repeated thrusting of the penis into the vagina, an activity that is often accompanied by involuntary vocalizations (e.g., grunting, moaning), heavy breathing, and an accelerated heart rate. This period of copulation is sometimes termed the plateau, and it precedes ejaculation.
The thrusting, moaning, and panting typically culminate, especially for the male, in orgasm. During orgasm, the male usually ejaculates, releasing semen from the urethra. Orgasm for both partners can also involve whole-body muscle spasms, more involuntary vocalizations, and a feeling of euphoria. While multiple orgasms are unusual for males, females may experience several orgasmic pulses, either with or without the aid of manual stimulation (e.g., hands or a vibrator).
Orgasm is followed by a refractory period, during which normal breathing and heart rate resume. This period is often marked by intense sensations of wellbeing and calm. In males, the end of the refractory period is delimited by the ability to have another erection.