A sexual fantasy is a fantasy of a sexual nature. A person may or may not wish to enact their sexual fantasies in real life; some may find their fantasies completely unacceptable — or even physically impossible — were they to be transposed into real life.

Sexual fantasies may involve any human sexual practice or paraphilia. Common sexual fantasies include:

adultery
sexual bondage
group sex
sexual domination and/or submission
masochism and/or sadism
rape fantasy
homosexual or lesbian fantasies
incest
pedophilia or ephebophilia
being cuckolded
various other paraphiliac activities
Sexual fantasies that were once common, but have become archaic include:

the pastoral fantasy
the sexual temptation or torture of Christian martyrs, many of whom resort to self-mutilation to preserve their virginity or chastity

Gender differentiation

Study suggests that distinct tendencies differentiate male and female sexual fantasies. These traits are hardly universal; just because a person happens to be male does not indicate he will have only "male" fantasies that night, let alone for the whole of his life. In general, however, male sexual fantasies tend to be shorter and imagistic, where female sexual fantasies tend to be longer and have more narrative, as well as a greater focus on the relationships between the characters in the fantasy. The essence of a "male" fantasy might be captured in a photographic moment or a seconds-long clip of film; a "female" fantasy might require pages of text to fully capture the key elements. Note that, contrary to popular expectation, there is little to no difference between the genders in the transgressive content of their fantasies; a female is just as likely to fantasize about group sex with an entire fraternity as a male is to fantasize about group sex with an entire sorority.

Many theories have been proposed to explain these relative gender differences, on both sides of the nature versus nurture debate. Theories that focus on nurture point to attitudes prevalent in Western societies, where men's sexual needs are assumed to be wholly physical, while women's are treated as both emotional and physical. Theories that focus on nature suggest that both males' and females' sexual fantasies, and their sexual behavior, are shaped by their respective hormones to reflect their roles in procreation: for men, the sex act may be over in a very short time, while for a woman, if the sex act leads to pregnancy, it is a months-long endeavor. As is often the case with competing nature vs. nurture theories, the likelihood is that both play a role.

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