Apparently my first comment was more or less correct. From a Wikipedia page titled "Baby Smurf", regarding a story by the same name:
During a blue moon night, a stork leaves a baby Smurf at the door of a Smurf house and leaves. The Smurfs quickly hand the baby to one another until he ends up with Smurfette. The next day, all the Smurfs converge to see the baby, save for Grouchy Smurf, who hates baby Smurfs. Brainy Smurf asks if the baby is Smurfette's, which offends her.
The baby is adopted by all Smurfs (save for Grouchy). Handy Smurf makes a bed for the baby, including a machine to swing it, but Papa Smurf opts for swinging the bed at hand. Greedy Smurf bakes a cake for Baby Smurf, but Smurfette tells him that Baby is too small for cakes. Other Smurfs wash Baby's dirty clothes. When Baby sees Grouchy passing, he tries to imitate his gruff expression, which seems to mellow Grouchy down, but he fakes apathy when Smurfette asks him to carry the baby. Brainy asks Papa Smurf where the baby Smurfs come from, but Papa's answer is so complicated that he still doesn't understand anything. At night, Baby Smurf's crying doesn't let any Smurf sleep.
The next day, the Smurfs prepare a party in Baby Smurf's honor, but the stork arrives with a note saying that Baby was a "delivery error" and must be taken away. Everybody is upset including Smurfette who goes into a non stop crying spree, but they don't have any option but return the baby. However, they soon find that Grouchy Smurf has escaped with the baby. The Smurfs seek for Grouchy and Baby throughout the forest. Grouchy manages to avert being found, but then he decides to return to the village after noticing the many dangers Baby would face in the forest as well as Baby Smurf falling ill by catching a cold.
After the Smurfs give Baby back to the stork, life in the village turns sad; even Jokey Smurf and the Smurf getting his trademark explosive present try to laugh but end crying. That same night, the stork returns with Baby, with a note saying that Grouchy Smurf's care for the baby has caused the baby to be allowed to stay with the Smurfs.
Since the baby was a surprise, it would appear that there was no sexual activity involved, but Smurfette getting offended when Brainy asks if it is her baby suggests that the Smurfs know how reproduction usually works.