In genetic sex-determination systems, an organism's sex is determined by the genome it inherits. Genetic sex-determination usually depends on asymmetrically inherited sex chromosomes which carry genetic features that influence development; sex may be determined either by the presence of a sex chromosome or by how many the organism has. Genetic sex-determination, because it is determined by chromosome assortment, usually results in a 1:1 ratio of male and female offspring.
As I was reporting this piece, quite a few people told me that they were taking a break from sex and dating. This tracks with research by Lucia O’Sullivan, who finds that even after young adults’ sex lives start up, they are often paused for long periods of time. Some people told me of sexual and romantic dormancy triggered by assault or depression; others talked about the decision to abstain as if they were taking a sabbatical from an unfulfilling job.
Many critiques of online dating, including a 2013 article by Dan Slater in The Atlantic, adapted from his book A Million First Dates, have focused on the idea that too many options can lead to “choice overload,” which in turn leads to dissatisfaction. Online daters, he argued, might be tempted to keep going back for experiences with new people; commitment and marriage might suffer. Michael Rosenfeld, a sociologist who runs a longitudinal study out of Stanford called “How Couples Meet and Stay Together,” questions this hypothesis; his research finds that couples who meet online tend to marry more quickly than other couples, a fact that hardly suggests indecision.

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.[63] 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.[64] 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.[65]

Biologists studying evolution propose several explanations for why sexual reproduction developed and why it is maintained. These reasons include reducing the likelihood of the accumulation of deleterious mutations, increasing rate of adaptation to changing environments,[11] dealing with competition, DNA repair and masking deleterious mutations.[12][13][14] All of these ideas about why sexual reproduction has been maintained are generally supported, but ultimately the size of the population determines if sexual reproduction is entirely beneficial. Larger populations appear to respond more quickly to some of the benefits obtained through sexual reproduction than do smaller population sizes.[15]
Hinduism emphasizes that sex is only appropriate between husband and wife, in which satisfying sexual urges through sexual pleasure is an important duty of marriage. Any sex before marriage is considered to interfere with intellectual development, especially between birth and the age of 25, which is said to be brahmacharya and this should be avoided. Kama (sensual pleasures) is one of the four purusharthas or aims of life (dharma, artha, kama, and moksha).[86] The Hindu Kama Sutra deals partially with sexual intercourse; it is not exclusively a sexual or religious work.[87][88][89]
Sexual dimorphisms in animals are often associated with sexual selection—the competition between individuals of one sex to mate with the opposite sex.[42] Antlers in male deer, for example, are used in combat between males to win reproductive access to female deer. In many cases the male of a species is larger than the female. Mammal species with extreme sexual size dimorphism tend to have highly polygynous mating systems—presumably due to selection for success in competition with other males—such as the elephant seals. Other examples demonstrate that it is the preference of females that drive sexual dimorphism, such as in the case of the stalk-eyed fly.[43]
Humans, bonobos and dolphins show cooperative behaviour. In many cases, this behaviour has shown better results than what an individual can achieve alone. In these animals, the use of sex has evolved beyond reproduction and has taken additional social functions. Sex reinforces intimate social bonds between individuals. Overall, such cooperation also benefits each member of the group in that they are better able to survive.

At least for humans, this most basic of acts is anything but basic. As the pioneering sex researcher Alfred Kinsey put it, the only universal in human sexuality is variability itself. From attraction to action, sexual behavior takes many forms. Human interest in sex is not a matter left to chance but more a built-in imperative; survival of the species depends on it. The decks are stacked in sex's favor, as a passport to bonding, to intimacy, to pleasure, and even to human growth and healing. Bodies and interests change over the course of time, and the complexities of physiology and psychology mean that most people experience a sexual problem at some point in their lives. Although sex can be one of the most difficult topics for partners to discuss, it's one that also stands to draw couples closer together. The moral and political implications of sex vary greatly from culture to culture, and even within cultures and over time; still, there is agreement on one certainty: It's why we're alive today and what future generations depend on.