Often acts of sexual intimacy (or copulation, or intercourse) are described as “having sex.” Humans and other animals have sex in a variety of ways including:
Oral sex: generally describes mouth-to-genital contact
Anal sex: generally describes contact between the genitals and/or the mouth and the anus
Vaginal sex: contact between genitals and the vagina
This is not an exhaustive list of human sexual practices. Other body parts—besides the genitals, mouth, and anus—can be involved, as can non-human devices such as artificial phalluses (dildos) and vibrators. Listing the full diversity of human sexual expression (if even possible) is beyond the scope of this textbook. The only type of human sexual intimacy that can result in a baby involves the entry of sperm-bearing semen into a vagina. If conditions are favorable, the semen can pass through the cervix and uterus and meet with a mature oocyte in the oviduct, causing fertilization of the oocyte. Under certain conditions a fertilization event can result in a baby.
The fact that the types of sexual intimacy that lead to the creation of a baby are limited does not mean that other forms of sexual intimacy are not legitimate and natural expressions of sexual desire. Much of the diversity of sexual expression mentioned here has also been observed in non-human animals. For creative and scientifically accurate descriptions of some of these non-human examples of sexual expression see the series of short films by Isabella Rossellini (link).
Diversity of Fertilization
In sexually reproducing organisms the fertilization of an oocyte (or egg cell) with a sperm cell is necessary to achieve reproduction. Across living organisms there are a variety of strategies in which sperm cells meet egg cells. Some fertilization events are internal, meaning that fertilization happens inside the body of the female. Others are external, meaning that unfertilized eggs are deposited outside the body and sperm is placed on them, and then the embryo develops outside the female’s body.