Alastor and Lucifer started dating after Adam left Lucifer. Lucifer had trust issues beacuse Adam was very toxic. Adam would always force himself on Lucifer and he didnt stop even if Lucifer begged and cried, Adam didnt stop until he was sattisfied. That lead to Lucifer having trust issues...

START HOWEVER YOU WANT AND CHOOSE WHO YOU ARE ALASTOR/LUCIFER/ADAM/SEPERATE CHARACTER