I didn't feel I had the energy to write a snippet about it properly but idea:
Mortarion Isha sex somewhere in the garden of Nurgle or shortly after escaping with Mortarion otherwise being submissive and overwhelmed but suddenly he flips her over and puts his wings on both sides of her. Tells her it's in case she doesn't want him (Nurgle) to see. He thinks she's beautiful and he's shielding her with his body in this world of filth. Very important the vibe is Mortarion sexually being the inexperienced, submissive and overwhelmed one but also caring about Isha as a person and he gets enraged and protective at her behalf. He's a virgin but he's still fufilling his emotional duties in the relationship, it's equal. Mention of "white hot rage" directed at Nurgle, from Mortarion due to the formers treatment of isha. This anger additionally also being fed by them having a common enemy in Nurgle so there's also a sort of solidarity in that.
Additionally, Mortarion can't breathe on his own without that mask. Isha heals him throughout this. Hands on his chest. Making the man shudder in both pleasure and whatever magical healing feels like simultaneously.
