Love is any of a number of emotions and experiences related to a sense of strong affection or  profound oneness. Depending on context, love can have a wide variety of intended meanings. Romantic love is seen as a deep, ineffable feeling of intense and tender attraction shared in passionate or intimate attraction and intimate interpersonal and sexual relationships. Love can also be conceived of as Platonic love, religious love, familial love, and, more casually, great affection for anything considered strongly pleasurable, desirable, or preferred, including activities and foods. This diverse range of meanings in the singular word love is often contrasted with the plurality of Greek words for love, reflecting the concept’s depth, versatility, and complexity.

Sex refers to the male and female duality of biology and reproduction. Unlike organisms that only have the ability to reproduce asexually, sexed male and female pairs have the ability to produce offspring through meiosis and fertilization. The two sexes attract one another and communicate their readiness to procreate through differences in their biology.

An organism’s sex category reflects its biological reproductive
function rather than its sexuality or other behavior. The female sex is
defined as the one which produces the larger gamete and which typically bears the offspring.
In contrast, the male sex has a smaller gamete and rarely bears
offspring. In some animals, sex may be assigned to specific structures
rather than the entire organism. Earthworms, for example, are normally hermaphrodites.

Animal species

Main articles: Animal sexuality and Mating system

Animal sexual behavior takes many different forms, even within the same species. Researchers have observed monogamy, promiscuity, sex between species, sexual arousal from objects or places, rape, necrophilia, sexual orientation (heterosexuality, homosexuality, bisexuality and situational sexual behaviour) and a range of other practices among animals other than humans. Related studies have noted diversity in sexed bodies and gendered behaviour, such as intersex and transgender animals.

The study of animal sexuality (and primate sexuality especially) is
a rapidly developing field. It used to be believed that only humans and
a handful of species performed sexual acts other than for procreation,
and that animals’ sexuality was instinctive and a simple response to
the “right” stimulation (sight, scent). Current understanding is that
many species believed monogamous have now been proven to be promiscuous
or opportunistic in nature, a wide range of species appear to both masturbate and to use objects as tools
to help them do so, in many species animals try to give and get sexual
stimulation with others where procreation is not the aim, and
homosexual behavior has now been observed among 1,500 species, and in
500 of those it is well documented. There are species that have 3, 4 or
5 sexes. For example:

• the clam shrimp Eulimnadia texana has three sexes, that is, it is a triploid organism.^[1]
• harvester ant genus Pogonomyrmex has three sexes^[2] or maybe four sexes^[3]
• the reptile tuatara might have four sexes [1]
• Coprinus macrorhizus (Pers.) Rea might have three or four sexes [2]
• Coprinus lagopus has four sexes [3]
• Fausto-Sterling proposes that there are five sexes of humans ^[4][4]
• Other species have exhibited evidence of 5 sexes [5]

Humans

Stylised illustration of humans on the Pioneer plaque, showing both male and female.

See Human sexuality for information about sexual activities, sexual sensation, sexual gratification, and sexual intimacy between human beings

In humans, “sex” is often perceived as a dichotomous state or identity for most biological and social purposes - such that a person can only be female or male.
But many factors, including one’s biology, environment, psychology and
social context, have a role in determining how a particular person, and
those around them, view their sex. Although the table below shows
common differences between males and females, many people do not
correspond to “male” or “female” with regard to every criteria.

“Primary” sexual characteristics are typically present at birth and directly involved in reproduction. “Secondary” sexual characteristics typically develop later in life (usually during puberty) and are not directly involved in reproduction. Differences between the sexes are known as sexual dimorphism. At the biological level these differences are usually:

The relationship between the various levels of biological sexual
differentiation is fairly well understood. Many of the biological
levels are said to cause, or at least shape, the next level. For example, in most people, the presence of a Y chromosome causes the gonads to become testes, which produce hormones that cause the internal and external genitalia to become male, which in turn lead parents to assign ‘male’ as the sex of their child (assigned sex), and raise the child as a boy (gender of rearing).
However, the degree to which biological and environmental factors
contribute to the psychosocial aspects of sexual differentiation, and
even the interrelationships between the various psychosocial aspects of
differentiation, is less well understood as illustrated by the ongoing nature versus nurture debate.

Discordance

See also: homosexuality, cross-dressing, transvestism, transgender, and third gender

Discordance is the term used to describe the extent to which
people differ from the usual biological and psychosocial types
described above. Some discordances are biological, such as when the sex
of the chromosomes (genetic sex) does not match the sex of the external genitalia (anatomic sex) - this is known as intersex. Discordances between the biological and psychosocial levels (such as when the gender identity does not match the anatomic sex) or between the various psychosocial levels (such as when the gender role does not match the gender identity) are even more common, but less well understood.

In gender theory, the term “heteronormativity”
refers to the idea that human beings fall into two distinct and
complementary categories, male and female; that sexual and marital
relations are normal only when between two people of different genders;
and that people should follow roles determined by their gender.
Instead, some people have sought to define their sexuality and sexual
identity in non-polar terms, in the belief that the simple division of
all humans into “males” and “females” does not fit their individual
conditions. A proponent of this movement away from polar oppositions, Anne Fausto-Sterling, recognized five sexes: male, female, merm (male pseudohermaphrodite), ferm (female pseudohermaphrodite) and herm (true hermaphrodite).
Although she was heavily criticized, her idea demonstrates the
difficulty and imperfection of the current social responses to these
variations.

Social and legal considerations

Forms of legal or social distinction or discrimination based on sex include sex segregation and sexism.
Notably, some businesses, public institutions, and laws may provide
privileges and services for one sex and not another, or they may
require different sexes to be physically separated. Recently, western
societies have moved towards greater sexual equality.

In gender theory, the term “heteronormativity”
refers to the idea that human beings fall into two distinct and
complementary categories, male and female; that sexual and marital
relations are normal only when between two people of different genders;
and that each gender has certain natural roles in life.