Matthew Scott and Vanessa James having sex in Icarus base.
In biology, sex is a process of combining and mixing genetic traits. Often it results in the specialization of organisms into a male and female variety, and is how most races reproduce.
Variation among races
While Goa'uld are agender, there are some Goa'uld who can produce larva. These rare Goa'uld are referred to as Queens and are capable of spawning numerous larval Goa'uld symbiotes (called prim'ta) in each cycle.
Among the Goa'uld, it was taboo for two Goa'uld hosts to have unprotected sex as it would lead to the birth of a harcesis - an individual who would possess all the knowledge of the Goa'uld who fathered it. With this knowledge, a harcesis could easily jeopardize the Goa'uld power structure, such as by revealing that the Goa'uld are false gods to their followers or by divulging the secrets of Goa'uld technology. For this reason, known harcesis were hunted down and killed with impunity. (SG1: "Maternal Instinct")
The Asgard were one notable race who, instead of utilizing sex, reproduced through cloning. (SG1: "Revelations", "Unending")
History
In 2009, Matthew Scott and Vanessa James had sex in a closet in Icarus Base. (SGU: "Air, Part 1")
Later that year, Colonel Everett Young (while in the body of Colonel David Telford) had sex with his wife Emily Young via the Ancient Long-range communication device. Due to Destiny dropping out of FTL while this was transpiring, Colonel Telford momentarily switched back to his own body in the middle of intercourse, leaving him utterly shocked. (SGU: "Earth") Ronald Greer is known to have had sex with Lisa Park, and Eli Wallace is believed to have had sex with Ginn. (SGU: "Life", "The Greater Good")
