I finally did it.
I finally came up with the reason Emma Frost betrays us all and hooks up with the universally hated Scott Summers, despite it being totally against her character and disgusting.
Emma Frost and Jean Grey have a central thing in common. They both have hosted the Phoenix Force. This would necessarily have profound effects on their physical brains.
That Asshat Scott merged with the entity of Apocalypse. I didn't pay much attention to this as I spend most of my time pretending that they killed him off ages ago. BUT it serves my theory well because it is this: the Phoenix Force broke Emma's brain.
Well, I'm thinking there's a magnetic polarity thing occurring wherein Emma's brain points towards Scott's brain, so it's not actually HER deciding to hook up with Scott, but rather an instinctive alignment towards the only other body that has experienced the same kind of physical possession.