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