My (controversial) Shane and Ilya...
They're both secret boob guys
We all know they love ass - can you blame them? Look at the material - and they both love squeezing each other's pecs
MyIlya loved tits during his fuckboi era. Big, small, high, low, in, out he loved a pair of boobs to bounce and swish and play with and cum on. He was sticking his face in a woman's chest any chance he got. Boobs were his thing.
Shane on the other hand... It's more complicated. Sex with Rose was torture and humiliating and something Shane still has nightmares about, but he was so fascinated with her boobs. They were so much softer than he had imagined, and her nipples had no hair around them. Her boobs moved differently than pecs. Sometimes, he misses that relaxing softness in his hands.
I don't think Shane and Ilya ever admit their love for boobs, and if they do, it's much later into their marriage. And they both have a laugh about it. And then maybe Ilya buys boob shaped stress balls for the both of them. And maybe they're both a little to into mindless mushing them around during idle times.