I will start with the cynical obvious.

Magneto and Xavier

Others I can remember?

Jean and Ororo

Cable and Deadpool

Rogue and Psylocke

Wolverine and Cyclops

Gambit and Storm

Jubilee and Husk

Beast and Jean

Wolverine and various x-men females (Rogue, Kitty Pryde, Jubilee, Rogue, X23, Storm and Jean)

Kitty Pryde and Illyana Rasputin

Domino and Cable

Cyclops and Emma Frost

Wolverine and Nightcrawler