In this universe, the habits and social implications of eating and having sex are swapped. It becomes extremely intimate to eat, it is done alone or with a chosen and loved partner, or in more taboo places such as restaurants, seen in this universe like libertain clubs. Eating is a pleasure and sharing food related past experiences not common. Meanwhile, sexual intercourses are a common thing to do, at least three times a day, when people would eat in real life (in the morning, at noon and in the evening). It can be done alone, with other known people or strangers. It may be done quickly before work or in commute, or the characters might like to take their time and share a good fuck with friends. Other small "treats" are common during the day. Sex is therefore a usual and easy talking subject. It is not weird to masturbate in front of other people or with other people. This way of things is normal and not questionned by characters. You can invent more fantasy as long as you keep the idea of switching sex and eating, and don't make the characters question this swap of things. It is normal for the characters to live this way. The erotic story can describe both normal life where fucking three times a week is a norm or sexy eating moments.