I usually play as male unit and have chrom marry sumia. Sumia's marriage options are very limited unfurtunately. It's Chrom, Frederick, Gaius, Henry and that's it.
It all depends on how you want to class her daughter Cynthia. Frederick if you want her to be a knight, or to have higher defense/attack stats. Gauis if you want her to have the option to be a thief, or to pass the Locktouch skill to Cynthia. Henry if you want Cynthia to be a mage, or have higher mage stats for Dark Flier. All depends on what you want for Cynthia.