Stories set in the universe where women regress into 12-year-olds every month
Chapter Description: Introduction to the Universe
These stories are set in an alternate universe where women, inside of having their menstrual cycle, have a "heat" cycle instead. So instead of suffering a painful menstruation each month women instead go into "heat" slowly physically regress to the age before puberty (usually around 12 years old) and then stay that way for a whole week until they slowly grow back to their original age. During their heat period their libidos would rise to really high levels, which is usually why women in heat would have to take time off from their work/school to quench their libido usually with the help of their partner(s).
Sex in this universe therefore happen a lot more and are a lot more casual/less frowned upon. Public sex with women in heat are also accepted in several places. There is no risk involved in this as the youthened bodies of those in heat are incapable of getting pregnant.