By default Chrom becomes wed to Sumia relatively early in the game. Marriage has the support rank of "S" on the Support menu. When inspecting Chrom's available support options I saw that there are 3 other possible "S" characters. Can Chrom marry someone other than Sumia?
Yes. The tricky part is is that Chrom needs to have formed a relationship after chapter 11. At that point Chrom will marry one of; your Avatar (if female), Sully, Maribelle, Sumia, or Olivia. Whichever of those 5 he has the highest relationship with, and is not already married off. If he married Sumia in your game(s), it is simply because he has spent more time gaining relationship ranks with her in battles than his other suitors.
Olivia is the trickiest to get him to marry, as you have but one mission to recruit and have Chrom woo her (just have them constantly support each other in battle, and have her dance for Chrom every turn).
I once had to re-start, because while I was trying to get my avatar to marry Donnel, Chapter 11 would have pushed them to S-rank, but because my Avatar was A-rank with Chrom, it forced the marriage between Chrom and my avatar after Chapter 11.
Interestingly, if all of Chrom's suitors are married off and Olivia has no contact with him in chapter 11 (or she dies), Chrom will marry some random village girl.
(Chrom is the only character who's marriage is determined at a fixed point. All other characters can be married off by having them support each other in battle at your leisure)
Chrom can only marry Sumia, Sully, female Robin, Maribelle, or Olivia. It goes the same way like other supports. So, just pick one of the units he is able to marry, and it doesn't HAVE to be Sumia; Sumia is just considered to be Chrom's "canon wife". Just pick a unit (by your question, I assume you don't want Sumia, so don't pick Sumia) that Chrom CAN marry (besides Sumia) and get them to S-support or highest level support that you can. If Chrom isn't married while trying to get an S-support with one of the units, at the end of Chapter 11 there will be a bonus scene where he will marry the unit he has the most support points with. I just made female Robin (my avatar, yes I'm a girl) stick with Chrom since the beginning of the game and I got an A support by the end of Chapter 11, so I married Chrom. So yes, Chrom can marry other female units, just not every single female unit in the game that appears before Chapter 11. If Chrom does not get at least nine support points with his possible spouses, then he will marry a nameless village maiden, who can't contribute to the Shepherds, so I recommend making Chrom have a relationship with his possible wives since the village maiden is a stupid (and completely useless) idea.