Aztec sexuality and their cultural attitudes toward it were fairly repressed. They maintained strict mores regarding intercourse, and any related actions outside of their concept of marriage were not only forbidden but were often cause for capital punishment. Despite this reluctance, sex work, one of the oldest and most widespread professions in the world, was common in Aztec culture. Their understanding of the practice was far different from that of the modern western world, however; within Aztec society, such work was very ritualistic and — within those confines — accepted in mainstream culture.
Conversely, homosexuality in the Aztec empire was strictly prohibited and, despite being almost unheard of due to repressed attitudes, was an offense often resulting in capital punishment. Though many relevant sources on the topic of Aztec sexuality are unreviewed, the information that is available shares many prudish aspects of sex in the Aztec Empire.
In a Reddit AMA, user 400-Rabbits moderated a panel about Aztecs; he described himself as a “four-field anthropologist,” and led a discussion on Aztec sexuality.
Intercourse outside of marriage was frowned upon, but it was used ritualistically, especially in sacrifice. Within the thread, 400-Rabbits said a male sacrifice who spent a year as an avatar (or ixptla) of the god Tetzcatlipoca, who created the world with Quetzalcoatl, would be given four wives one month before being sacrificed.
Female sacrifices suffered a more horrific fate: a woman acting as an avatar for the mother goddess, Toci, would be handed over to the tlatoani (roughly translated as "king" or "ruler") as concubines prior to her execution.
Her end was also gruesome: the priest impersonating Toci in the sacrificial ceremony would flay — and then wear — her skin. Toci, as presented in the iconography of the time, was often pictured as wearing human skin.
Despite these views, references to "cross-dressing boys [concubines]" and their related duties hint toward a double-standard in regard to ritualistic sex practices.
Self-stimulation was considered unacceptable among the Aztecs. As Gary Jennings outlines in his book Aztec, those who were caught masturbating were punished by having ground chili peppers rubbed into their genitals.
This punishment is starkly contrasted with some other cultures' practices: some Native Americans purposefully rubbed hot peppers on their genitals to prolong pleasure during intercourse.