Why Marry? Because she is Queen regent and a prominent member of an extremely powerful family—marrying her could be an advantageous political move.
Why F***? Because she is one of the fairest ladies in the land.
Why Kill? Because the Maester General has determined that being married to Cersei is hazardous to your health. And unless you're Jaime, you are not her bedroom type. Also: she's Cersei Lannister. Come on now.
Why Marry? Because even though Lysa is a neurotic psychopath, she’s pretty easily manipulated if you play your cards right (though you have to be okay with playing second fiddle to Little Lord Robert). Also, as she’s Catelyn’s sister, you would have a good excuse to go hang out with the Starks if you wanted to!
Why F***? Because Lysa is completely love-starved and all that longing would probably make for a fun time.
Why Kill? Because the woman is completely insufferable.
Petyr "Littlefinger" Baelish
Why Marry? Because he has worked his way up from a backwater lordling to one of the most powerful men in Westeros: this guy is a master player and allying yourself with him may not be a bad bet. If you truly believe yourself to be in his good graces his scheming could get you places.
Why F***? Because he’s a quick study, and has probably picked up a few good tips in the brothel he owns.
Why Kill? Because as Ned Stark learned too late, Littlefinger is dangerous. If you leave him alive, you will always have to be on your toes.
Cedar: Marry Cersei, F*** Lysa, Kill Littlefinger
Erin: Marry Lysa, F*** Cersei, Kill Littlefinger
Jamie: Marry Lysa, F*** Cersei, Kill Littlefinger