I think these two deserved more backstory than they got in Midnight Mass. So here's my naughty little idea on how things may have been between them when they were young. Be advised, this fic is rated at a hard M for explicit sex. Things get very spicy so proceed with caution if you're squeamish. Otherwise, enjoy!
"The mass has ended, go in peace."
"Thanks be to God," Mildred Gunning murmured with the rest of the congregation. The mass had been difficult, seemingly endless. The coil of desire, sitting heavy and tight in her stomach had given her no peace. John was suffering as well. She could see it in his eyes: sadness, guilt…and something else…something darker. Anger, she decided. It was nothing the other churchgoers would notice. It was only meant for her. And anyway, he was such a passionate preacher that any emotion he betrayed could easily be interpreted as religious fervor. But it was there. And strangely, it aroused her all the more. She wanted that anger turned on her. She wanted the fire in John's eyes to burn her, mark her. She wanted it to hurt.
She touched George's shoulder, "Hey, do you mind if I stay for a minute and take confession? It's been too long."
"Yeah, that's ok. I'll wait for you and we can walk home together."
If he was at all put out by his wife wanting to linger in church rather than spend every moment they could together, he did not show it. Nor was he suspicious. He'd already anticipated the request when he'd seen her pointedly stay in her seat instead of receiving communion.
Millie stood aside so George could scoot past her. She put a bland, patient expression on her face and watched as everyone slowly made their way out. Waiting, waiting.
She finally let herself walk up to the priest as he was saying his last "Good afternoon!" and waving the stragglers through the church door.
"Father Pruitt, I know it's not your usual hours, but would you be available for a quick confession?"
He turned his beautiful, dark eyes on her. A shudder involuntarily ran up her spine. Anger. Challenging, erotic. Smoldering under a pleasant social smile.
"Of course, Mrs. Gunning. I have the time. Come inside."