In most birds, both excretion and reproduction is done through a single posterior opening, called the cloaca—male and female birds touch cloaca to transfer sperm, a process called "cloacal kissing". In many other terrestrial animals, males use specialized sex organs to assist the transport of sperm—these male sex organs are called intromittent organs. In humans and other mammals this male organ is the penis, which enters the female reproductive tract (called the vagina) to achieve insemination—a process called sexual intercourse. The penis contains a tube through which semen (a fluid containing sperm) travels. In female mammals the vagina connects with the uterus, an organ which directly supports the development of a fertilized embryo within (a process called gestation).
In the past[when?], children were often assumed not to have sexuality until later development. Sigmund Freud was one of the first researchers to take child sexuality seriously. His ideas, such as psychosexual development and the Oedipus conflict, have been much debated but acknowledging the existence of child sexuality was an important development. Freud gave sexual drives an importance and centrality in human life, actions, and behavior; he said sexual drives exist and can be discerned in children from birth. He explains this in his theory of infantile sexuality, and says sexual energy (libido) is the most important motivating force in adult life. Freud wrote about the importance of interpersonal relationships to one's sexual and emotional development. From birth, the mother's connection to the infant affects the infant's later capacity for pleasure and attachment. Freud described two currents of emotional life; an affectionate current, including our bonds with the important people in our lives; and a sensual current, including our wish to gratify sexual impulses. During adolescence, a young person tries to integrate these two emotional currents.
The pleasure of sex arises from many factors including the release of neurochemicals such as oxytocin and dopamine, which flood the system during orgasm, as well as the sense of connection communicated by touching, massaging, and cuddling. Given the enormous variability in activities that people find arousing, there is no one way to be sexual. Men are especially stimulated by visual imagery, and about 90 percent of young men report using pornography with some regularity, often because they lack a partner or don't know how to bridge the differences in sexual appetite and interest that can occur between partners in the absence of discussion of their sexual pleasures. Many people engage in behaviors that were once perceived as atypical, such as dominance play and anal intercourse. Researchers know that flexibility in sexual repertoires is healthy and generally enhances relationships; they regard a specific behavior as problematic only when it creates harm or distress for one or both partners or when the behavior is compulsive—that is, it becomes the only means of arousal. "Sex addiction" is a label often used to suggest excessive interest in sex, but studies show it may be more related to the moral/religious environment in which a person lives.