How far are you into the game?
[spoiler="Spoilers for Post Chapter 11"]If you don't Marry Chrom off, he will get Automarried after beating Chapter 11, if the Four potential canidates (Sumia, Sully, Maribelle, and Olivia) are either dead, married off, or don't have a high enough relationship value, Chrom will get married to a nameless NPC, this is bad. If Sumia is available and has tied Support with Sully or Maribelle (Olivia you get in Chapter 11, so she's can't get support at all) She will be chosen. If you want him to marry Olivia, you need to marry off the other three before Chapter 11 AND make Olivia dance for Chrom at least 3-4 times so you don't get Random Village Maiden.[/spoiler]