Menage Lost — Cynthia Sax

Trapped on a plant-dominated planet, Mirian hasn't seen another humanoid in five years. She's never seen humanoids resembling Furu and Xan, a beautiful aristocrat and his beastly warrior lover. Watching their shared passion makes her burn with desire and ache for companionship. Wishing to experience the ecstasy of their touch, Mirian eagerly trades the only thing she owns -- her body -- for what she believes to be a breathtakingly ardent but casual affair.

Furu and Xan don't engage in casual affairs. They want Mirian permanently as their breeder, and they'll battle their unnatural female, a male-eating plant and the entire Balazoid army to claim her.