Sexual intercourse, in its biological sense, is the act in which the male reproductive organ (in humans and other higher animals) enters the female reproductive tract, called copulation or coitus in other reference.  The two entities may be of opposite sexes or they may be hermaphroditic as is the case with snails.
 
Traditionally intercourse has been viewed as the natural endpoint of all sexual contact between a man and a woman, and is commonly confined to this definition today. The meaning of the term, however has been broadened in recent years and now labels at least three different sex acts. These three types of intercourse are: vaginal intercourse involving vaginal penetration by the penis oral intercourse involving oral caress of the sex organs (male or female) and anal intercourse involving insertion of the males penis into his partners anus.
 

Sex acts that involve digital (use of fingers or hands) intercourse or mutual masturbation are more often referred to as outer course (with oral sex at times listed as an aspect) while the term sex in the context of sexual intimacy is often understood more widely to include any mutual genital stimulation.

 

Vaginal sexual intercourse also called coitus is the human form of copulation. While its primary purpose is reproduction it is often performed exclusively for pleasure and/or as an expression of love and emotional intimacy. Sexual intercourse typically plays a powerful bonding role; in many societies it is normal for couples to have frequent intercourse while using birth control sharing pleasure and strengthening their emotional bond through sex even though they are deliberately avoiding pregnancy.

 

Sexual intercourse may also be defined as referring to other forms of insertive sexual behavior such as oral sex and anal intercourse. The phrase to have sex can mean any or all of these behaviors as well as other non-penetrative sex acts not considered here.

 

Coitus may be preceded by foreplay which leads to sexual arousal of the partners resulting in the erection of the penis and natural lubrication of the vagina.

 

To engage in coitus the erect penis is inserted into the vagina and one or both of the partners move their hips to move the penis backward and forward inside the vagina to cause friction typically without fully removing the penis. In this way they stimulate themselves and each other often continuing until highly pleasurable orgasm in either or both partners is achieved. Penetration by the hardened erect penis is also known as intromission or by the Latin name immissio penis (Latin for "insertion of the penis").

 

Oral sex consists of all the sexual activities that involve the use of the mouth tongue and possibly throat to stimulate genitalia. It is sometimes performed to the exclusion of all other forms of sexual activity. Oral sex may include the ingestion or absorption of semen or vaginal fluids.

While there are many sexual acts involving the anus anal cavity sphincter valve and/or rectum the specific meaning of anal sex is the insertion of a mans penis into another persons rectum.