Rymellan 1 by Sarah Ettritch

Lesley and Mo can't imagine life without each other. If it were up to them, they'd settle down, raise daughters, and lead happy, fulfilled lives. But they live on the planet Rymel, in a strict society that selects life-mates for its citizens and executes those who violate their life-bonds. Girlfriends since their teens, Lesley and Mo know they should break up but can't let each other go. They dread the day the state summons them to meet their selected mates. Meet Lesley and Mo when they're young adults in love and follow them until their time together runs out. Will they do what their society expects of them, or will they sacrifice their lives for their love?