Twilight + Rainbow These two are like an old married couple, and love each other deeply and sweetly. They enjoy long flights at night, reading together, and picking up hot young ladies on a Friday night. They are a jock/nerd power couple, and are excellent parents. Twilight and Dash are perfect for each other, and balance out the others faults.
Rainbow + Flutters Their love is sickly sweet, all flowery poetry and picnic dates. Fluttershy is constantly showered with affection, always giggling sweetly when dashie presents her with a bunch of flowers with the roots still attached. As a pair, they aren't the best for discipline, but both love to show their foals new things and teach them through experience.
Twilight + Flutters Their love is all passion and heat, and while they work as a couple, their relationship is rooted in physical attraction. When Dashie isn't around to play middle-mare, these two are like two rabbits, and the only biologically conceived child in the household is a product of their lovemaking. They work very well as parents though, they have the patience to have hard conversations with their foals, and teach them the complicated parts of life.
I want to make it very clear, that Twilight and Fluttershy do in fact love each other very much, and enjoy being married and raising a family together. but at the end of the day, their feelings for one another came to be after they started sleeping together. These things happen IRL and it is totally valid for a relationship to start this way.