Seme is a dream demon. Gets food/energy from nightmares and uke has 100% chance of having bad dreams
Seme gets possessed by an incubus that was near starvation. So uke ends up having sex with the seme to replenish the incubus's energy. Since the seme keeps fainting when starving
Uke is a succubus and needs male body fluids to survive. So his childhood friend seme (who is in love with him) helps out
Your Dream Is Delicious