Sex describes any process where genetic information is combined from different members of the same species in order to create offspring. It is practiced by the vast majority of biological organisms, including bacteria and plants.
In humans, sex is a complex part of behavior and humans often engage in sexual acts with no intention of procreation. Humans appear to be the only sexually active organisms that understand the connection between sexual activity and procreation, and have used this knowledge for millenia for animal breeding and family planning.
House, M.D. often explores the relationship between sexual activity and medicine. In additon to the fact some diseases can only be spread by sexual contact, sexual relationships often reveal important clues about a patient's environment or personality (such as the stress that can arise from hiding a relationship). However, patients often go to great lengths to hide such information, even from physicians, due to the prevalence of social norms and the need for privacy.
In addition, issues such as gender identification, sexual behavior and sexual preferences are often deeply examined within the episodes.