In the mouths of women (and poop there too)

"...in the mouths of women"

In this world, once women become adults, they develop a need to consume the waste products of others, in rare cases requiring it more than water. For the first week or so, it tastes about as horrible as you'd expect, but after that most women quickly get used to it and in many cases they eventually learn to love it and look forward to it.

Anyone who refuses to do their business in a ladies mouth is considered to be unusually cruel and repeat offenders can be imprisoned.