Sex is an intimate interpersonal activity among humans and various species of animals that is used for procreation, pleasure, and expressing love for one another. Within Christianity it is considered a privilege and a blessing for heterosexual married couples, as it is meant to unite the man and woman together as "one flesh". Outside of religion and marriage, sex tends to be used for mostly recreational purposes, though there are people who also use it for financial purposes.
Mainstream Smurfs Media
In mainstream Smurfs media, it is a topic that is generally avoided due to the characters and their stories being created mostly for children, and any mention of Smurfs engaging in this activity would most likely appear in "non-canonical" works of an adult nature. However, it can be assumed that the Smurfs do have some level of education on this topic when it comes to how humans and animals reproduce, since the Smurfs themselves can only reproduce through non-physical means.
Empath: The Luckiest Smurf stories
In Smurf culture
In the EMPATH: The Luckiest Smurf story series, the Smurfs do engage in this form of activity, though it mostly occurs with married couples, since Smurf culture emphasizes moral purity, marital fidelity, and the creation of families as virtues for the ideal use of sex, and the idea of same-sex unions is highly discouraged. Smurfs are considered physically developed and capable once they reach 100 years of age, although societal norms suggest that they must be at least 150 years old to marry. (The onset of reproductive ability for male Smurfs is indicated by the first occurrence of the "Smurfette dream".) However, the only known case of this activity happening out of wedlock was when Aristotle fathered Lillithina's second son Brainy to replace the assumed loss of her first son Empath after her separation from her husband Culliford. Whether there were any societal repercussions when this was made public knowledge, if it was even made known at all, is unknown.
The activity itself is euphemistically referred to as "getting under one's hat", a Smurf mating ritual that only occurs during a married couple's honeymoon period. Hefty assumes this is all that Smurfette really wants of Empath and believes that it is the reason she is attracted to him in the first place. Smurfette, however, had no idea what the term even meant until Empath showed her on the honeymoon, and has shown on numerous occasions that she loves Empath for many different things other than his physical appearance.
With the emergence of the Imaginarium, it is assumed that some level of experimentation as far as imagined intimate relations between the male Smurfs and a potential mate has started, although there isn't much evidence to support or disprove this idea. Papa Smurf during the events of "Days Of Auld Lang Smurf" has been seen by Empath with an Imaginarium-created version of his wife in a private moment together, although it was interrupted by Empath's appearance in the fantasy setting. However, Tapper, who is firm in his beliefs as a Christian, refuses to use the Imaginarium for that kind of experimentation, believing that it is only right to give himself fully to a mate in holy matrimony, and is eager to help his fellow Smurfs when it comes to dealing with temptation.
In the Mirror Universe Smurf culture, there are hardly any standards as far as the appropriate use of sex, as Smurfs of any age are free to engage in intimate relations with Smurfs of either gender. The only thing that its Papa Smurf keeps off-limits from his little Smurfs is having a marital relationship with its Smurfette, who he has married sometime after she was changed into a real Smurf. This universe's version of Mother Nature is worshiped by the Smurfs as a sex goddess, who allows for all forms of intimate activity to occur among living beings without any restraint.
In human cultures
Sex within human cultures in the time period of the series happens mostly with consenting adults that are either married or unmarried, though cultural norms may dictate its appropriateness. In the story "Smurfette's Inner Beauty", Hogatha the witch used Smurfette's beauty to make herself attractive in order to engage Harlequin the warlock in a one-night stand in her own castle which she barred to keep the Smurfs who were pursuing her from entering.
In other cultures
With other species such as wood elves, such form of intimacy would be expressed through the inborn technique of "dream sharing" with married couples. It is also the activity among bonded Psyches of the same gender, though it lacks any emotional romantic elements since the Psyche Master considers them to be a taboo in Psychelian culture. Nevertheless, Polaris Psyche is the only member of his people at the present point in the story series who has had any sort of physical intimacy with anyone outside his own race. In the story "Hefty, The Luckiest Smurf", Cassiopeia was forced by the Psyche Master to become Empath's mate in Psychelia in order to quell his desires.
Wartmonger culture euphemistically refers to intimate activity as either "stroking one's horns" or "getting nice and squirmy", although whether the activity leads to reproduction or not remains unknown. It is mainly engaged in through the female Wartmongers prostituting themselves to the males, since the males and females are segregated from each other in most societal activities, although there are rare cases of males and females being married to each other.
In the early part of The Lost Year, Empath and Polaris discovered a woodnymph brothel within a tavern that was being operated by Lilliput the barmaid, with Daffodil as one of the servant girls hired to provide entertainment for the patrons. Neither Empath nor Polaris engaged in any intimate relations with the servant girls during their stay.
The author of the story series prefers to keep any descriptions of close intimacy regarding this activity to a minimum because of the audience of readers.
This activity is only implied, if not directly mentioned, to have happened in the Glovey Smurf story series regarding the main character and his wife Smurfette and the subsequent birth of their son Angelo. It is however explained in the mini story, The Whole Tour that something did happen between both Glovey Smurf and Smurfette.
RDK Stories/Expanded AU
Humans reproduce through physical means and are able to procreate in heterosexual relationships, but homosexual relationships do not have the combined ability to do so. More coming later!
In the Smurf Village, reproduction is through non-physical means, so the act of intimate closeness is kept private between the two (or more) involved in the act, and there is no capability of procreation. Being a single-gendered race, males are sterile and females, being magically created, are likewise. In order to establish families with children, a male Smurf baby may be delivered to the specific Smurf(s) or they would have to use the Smurfette formula for a daughter.
Sprites (including Watersprites, Firesprites, Woodsprites, and Airsprites) have the capability of procreating through physical means, but they may prefer to reserve their magic and energy to bring a child into existence by combining both individual's efforts. More to come later!
Hero: The Guardian Smurf stories
The Smurfs in the series do engage in this ritual, but on strict terms that they must be married first before they engage in the ritual. It is performed by married couples, preferably a Smurf and a Smurfette, in order to consummate their marriage. However, sex isn't used as a means for procreation, as Mother Nature herself magically confers the blessing of fertility upon Hero's two wives of separate marriages as well as those of his fellow Smurfs that were created with the Mirror Of Opposition. In "A Talk With Mother Nature", she explains that since the Smurfs are by nature a one-gender race, the males lack the essential parts for reproduction, though she promises that in the event that she will pass away, she will magically grant the Smurfs the ability to reproduce without needing her to confer blessings of fertility.
In the story adaptation "The Grey Smurfs", the Grey Smurfs do not see a problem with either polygamy or bisexuality as far as the three-way relationship their Hero Smurf has with their Smurfette and their Wonder Smurfette, even though there are no other known intimate relations among the Grey Smurfs.