Taboo by Sestina, Nina

The government enacts free use to stimulate dwindling birth rates. Spencer and Kari find out from their parents that they have been selected as a free use household, effective immediately.

They go back to their shared room, and Spencer can't stop looking up and down her supple body, knowing that it now belongs to him.

Kari can feel the way he's looking at her, and gets all tingly and wet down there because she knows what's about to happen.