William Masters and Virginia Johnson described four stages in the human sexual response cycle: excitement, plateau, orgasm (which seems to involve similar feelings and brain activity in males and females), and resolution. During the resolution phase, males experience a refractory period in which renewed arousal and orgasm are impossible.
     Sexual dysfunctions are problems that consistently impair sexual arousal or functioning. They include erectile disorder and female orgasmic disorder, and can often be successfully treated by behaviorally oriented therapy or drug therapy. Paraphilias are conditions, which may be classified as disorders, in which sexual arousal is related to nonhuman objects, the suffering of self or others, and/or nonconsenting persons.

Safe-sex practices help prevent sexually transmitted infections (STIs). Condoms are especially effective in preventing transmission of HIV, the virus that causes AIDS. A vaccination administered before sexual contact can prevent most human papilloma virus infections.

External stimuli can trigger sexual arousal in both men and women. In experiments, depictions of sexual coercion have increased acceptance of rape. Men respond more specifically to sexual depictions involving their preferred sex. Sexually explicit material may lead people to perceive their partners as comparatively less appealing and to devalue their relationships. Imagined stimuli (dreams and fantasies) also influence sexual arousal.

Rates of teen intercourse vary from culture to culture and era to era. Factors contributing to teen pregnancy include minimal communication about birth control with parents, partners, and peers; guilt related to sexual activity; alcohol use; and mass media norms of unprotected and impulsive sexuality. High intelligence, religious engagement, father presence, and participation in service learning programs have been predictors of teen sexual restraint.

Sexual orientation is an enduring sexual attraction toward members of one’s own sex (homosexual orientation), the other sex (heterosexual orientation), or both sexes (bisexual orientation). About 3 or 4 percent of men and 2 percent of women in Europe and the United States identify as exclusively homosexual, and 3.4 percent of Americans describe themselves as lesbian, gay, bisexual, or transgender. There is no evidence that environmental influences determine sexual orientation. Evidence for biological influences includes the presence of same-sex attraction in many animal species; straight-gay brain differences; higher rates in certain families and in identical twins; exposure to certain hormones during critical periods of prenatal development; and the fraternal birth-order effect.

Scientific research on sexual motivation does not attempt to define the personal meaning of sex in our lives, but sex research and education are not value free.