@martinuzz: Neither of them will marry anyone. Both are purely hetero, but neither will commit to marriage.
I believe there are options for gaydar to change the orientation. Try "help gaydar" in the DFHack console.
Otherwise, the soul orientation flags are boolean (true/false) flags:
The first one is Indeterminate, which is True only on adventurers. Then you have romance_male, romance_female, marry_male, marry_female (probably with slightly different names and order (this is written from memory), and your characters have the romance_male and romance_female flags set, respectively. You want the marry_male and marry_female flags to be set instead, so the romance_ flags should be set to toggled to False, while the marry_ flags should be toggled to True.
None of the flags in a gender pair True: Uninterested in that gender.
Romance_ and Marry_ for the same gender will not be set at the same time.
A flag set for both genders means the character is some version of bi-sexual, while a flag only for the own gender means it's a case of some flavor of homo sexual.
Boolean values are toggled in gui/gm-editor by moving the cursor on top of them and then hitting <ENTER>. Values of other types instead bring up a dialog box asking you to enter a value.