Though it is a natural part of sexual activity, sexual penetration is unlawful in some circumstances. Sexual penetration is a medical term, but it is most commonly used in statute law in relation to unlawful sexual acts. Some jurisdictions use the term sexual penetration, while others use terms such as sexual intercourse or carnal knowledge. Other jurisdictions refer to some forms of penetration as "acts of indecency", or other terminology. Many modern criminal statutes use the term sexual penetration when referring to unlawful sexual acts because it is a broad term encompassing (unless otherwise qualified) any form of penetrative sex, including digital or with an object, and may involve only the most minimal penetration.
When a man inserts his penis into a woman's vagina, it is generally called vaginal or sexual intercourse. When a penis penetrates another person's anus, it is called anal sex or anal intercourse. Penetrative oral sex may involve penetration of the mouth by a penis (fellatio) or the use of the tongue to penetrate a woman's vagina or vulva (cunnilingus). The tongue may also penetrate the anus during anilingus, which is a form of oral and anal sex. If one or more fingers are used to penetrate an orifice, it is called fingering or digital penetration. The insertion of an object, such as a dildo, vibrator or other sex toy, into a person's genital area or anus may also be considered sexual penetration.
Penetrative sex crimes are generally considered more serious than non-penetrative sex crimes, and sexual penetration of a child even more so. A child below the statutory age of consent generally cannot consent to acts involving sexual penetration. In laws, the term sexual penetration is commonly used in relation to sex with children. Unlawful sexual penetration is generally an offense irrespective of how deep the penetration was and irrespective of whether ejaculation of semen took place.
Laws may distinguish particular forms of sexual penetration as part of the offense. For example, the law in the U.S. state of Oregon provides:
"Unlawful sexual penetration in the first degree" is a felony that occurs where the offender "penetrates the vagina, anus or penis of another with any object other than the penis or mouth of the actor", if the victim is "subjected to forcible compulsion", or is "under 12 years of age", or "is incapable of consent by reason of mental defect, mental incapacitation or physical helplessness"