Olivia has the highest priority between her, Sumia, Sully, Maribelle and FeMU.
If you have unlocked all of these character's C supports and viewed none of them, Chrom marrys Olivia.
If you watch ANY of these C supports before Chrom's marriage, he cannot marry Olivia because you are not given an opportunity to raise her support level ABOVE C in the Chapter where you both recruit and potentially marry her.
Well after this experience I went through: http://www.gamefaqs.com/boards/643003-fire-emblem-awakening/65577412
I really don't know to be honest. FeMU seems pretty damn high on that priority list because I danced to insane proportions and still married FeMU even though I never unlocked or watched any female C-support.
MH will have to wait and why do most GameFAQ'ers in the 3DS board have sex with their 3DS?
Sumia is easily one of the better units in the game at that time. To not use her, particularly on Lunatic, makes the game harder for no reason. Just pair Chrom with Fred or a MaleMU it really doesn't matter.
For in much wisdom is much grief, and he that increaseth knowledge, increaseth sorrow.