Human sexual activity is the manner in which humans experience and express their sexuality. People engage in a variety of sexual acts, ranging from activities done alone (e.g., masturbation) to acts with another person (e.g., sexual intercourse, non-penetrative sex, oral sex, etc.) in varying patterns of frequency, for a wide variety of reasons.
Sexual activity usually results in sexual arousal and physiological changes in the aroused person, some of which are pronounced while others are more subtle.
Sex embodies a huge range of physical, emotional, and psychological factors. It can be performed alone, or with others. Although sex is at its most biological level, performed for the purpose of reproduction, among modern humans sex is most often performed for its own enjoyment, and social benefits.
For simplicity, we'll use these terms for discussion here;
To understand many of the deeper discussions of sexual behavior, and the according psychological and brain-chemistry impacts, it's important to consider orgasm and ejaculation as two separate things, even in men.
To aid discussion, we discuss each aspect separately;
These different aspects combine to form a wide range of sexual practices. Here is a diagram of some sexual practices and how they relate to each other, including semen retention.