Bonding is another word for the formation and nurturing of an emotional connection. Most evolved animals, from birds and reptiles to human beings, practice bonding.
When human beings bond, their bodies produce significant quantities of oxytocin, which is known as the bonding and attachment hormone. Females naturally have higher quantities of oxytocin to help them connect with their babies.
Human beings may spend time bonding with their offspring, other family members, friends, and intimate partners. Bonding can occur through talking with people. It can also occur through physical contact, from simple touching and caressing to more sensual and erotic interactions including sexual intercourse.
Despite being a physical act, sexual intercourse is a valuable way to create an emotional bond. Whenever a person interacts sexually with someone else there is a release of oxytocin, which encourages emotional bonding. At the point of orgasm, the body releases an extra surge of oxytocin.
This is one reason why many people struggle to have casual sex. Even though sexual interactions may start out as casual, or lacking in emotion, chemicals beyond the participants' control work together to foster a bond between them, making things less casual quite quickly. Sex can cause bonding in relationships that are both healthy and unhealthy.