If this works, the now fertilized ovum sticks to the side of the uterus and the woman becomes pregnant. The semen, which has sperm in it (which is made in his testicles), moves to the Fallopian tubes, and if it finds an ovum on its way, it will try to fertilize it. It involves a man and woman having sex without birth control until the man ejaculates, or releases, semen from his penis into the woman's vagina. Sexual intercourse is the natural and most common way to make a baby. The sperm and ovum unite through fertilization. This behavior in the above mentioned animals is also presumed to be for pleasure, which in turn strengthens social bonds. In most instances, humans have sex primarily for pleasure. These animals include bonobos, dolphins, and chimpanzees which also have sexual intercourse even when the female is not in estrus, and to engage in sex acts with same-sex partners. In certain animals, sexual intercourse is not only used for reproduction, but has taken other functions as well. Most mammals only mate when the female is at the point of estrus, which is the most fertile period of time in its reproductive cycle. Two animals coming together for the purposes of sexual reproduction is called mating. Sex is usually teleiophilic (between adults). These behaviors are sometimes included in the definition of sexual intercourse. Humans sometimes engage in behaviors that do not include the penetration of sexual organs, such as oral intercourse or anal intercourse or by non-sexual organs ( fingering, fisting). Sexual intercourse may also be between individuals of the same sex. The two animals may be of opposite sexes or they may be hermaphroditic, as is the case with snails.

Sometimes sexual intercourse is called coitus or copulation and is more casually known as having sex or sleeping together.

People and animals that sexually reproduce use sexual intercourse to have an offspring. Sexual intercourse (or simply called sex) is the insertion and thrusting of a male's penis into a female's vagina.

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