The act where one inserts an object, mostly a penis, into the vagina or anus of a person, for sexual gratification.
More commonly referred to as fucking, or of copulation, this is where the vagina or anus is entered, by the male penis, for the purpose of sexual intercourse.
It does not always refer to a penis being the inserting object, and can refer to the use of other sexual aids, such as a dildo, bottle, finger, but the common goal, is for sexual arousal and satisfaction.
While the actual moment of entry can be painful, due to the constricting of the muscles around the anus, the primary goal is to enter the rectal cavity, to create a filling sensation, as well as to reach the male prostate gland, that stimulates the sexual pleasure.
Such deep entry, or 'penetration' can aid in the orgasm for the receiving person.
Most who prefer to be penetrated, are noted as being bottoms, or submissive's, though that is not always the case.
The constriction of muscles within the rectum, add to the pleasures of the penetrating partner, noted as the 'top' and creates a sense of friction that aids in their eventual orgasm, and ejaculation.
At the same time, the pulsing penetration stimulates the prostate gland, aiding to the sexual pleasures of the receiving party.
Various sized objects can also be used for penetrative sex, which can include household items, or specially designed sexual aids, such as dildos.
A more common term, intercourse, is used here.
It means simply, that one achieves sexual gratification internally, while outercourse generally would refer to sexual satisfaction, without any painful entry of either the vagina, or anus.
Practice (Associated Acts):
Pegging, which is the use of a strap on dildo for the purposes of entering another person, is generally associated with Lesbian sex.
However, it is also used by women, to enter their male partner, so that they can experience what their female partners experience.
It is still 'penetrative sex' for the purpose of sexual satisfaction.
Sexual penetration is generally considered to be the ultimate end of sexual relations, between partners, as this is usually when the person doing the penetration, ejaculates, or achieves their orgasm.
Shortly afterwards, the other partner generally reaches their climax as well, hence the reference to this being the final stage of sexual relations between parties.
Extreme Penetration takes the act further than what many consider normal.
This involves the stretching of the opening, beyond normal limits, to achieve an even greater sensation of both pain, and pleasure.
Double Stuffing, where two penises enter the cavity, is a form of this penetration, in that it stretched the lining to the point of near tearing.
Fisting is considered by many, to be another form of 'extreme penetration' due to the size of the arm, that also enters the cavity, again causing added stretching of the muscles, as well as pain upon entry.
Fingering and/or rimming are not considered 'extreme' but are still considered penetrative sex.
Depending on the size of the dildo, it can be classed as extreme penetration or not. The thicker and longer the dildo, the more likely it is to cause added stretching, and pain, which would make it extreme.