I dont have proof of this, and I dont know if its intentional but while sleeping with willow after inviting her, for some reason results in having sex with lilith, and i can repeat this action, and I haven't even unlocked anything with Lilith.
Yes, thats a morning sex bug where lilith's morning sex content isn't properly implemented, I'd avoid asking anyone to sleep with you for the time being. It has been fixed for later versions.