For getting every children in a single route, your best option is to make a second Corrin that's the opposite gender, and do some very specific set ups.
For gay marriage, I think someone said that no matter what, any gay marraige (ex: M!Corrin+Xander) locks them out of getting children, but I'm not so sure on that.
Also, has anyone made a picture of what Fate's decompressed save files look like in HxD, I'm trying to get a second Corrin that can support.
With some very tedious steps and workarounds on the GameData.bin file, you can have kids in gay marriages.
Say you're marrying Tsubaki and Saizou, both are male, locking you out of both Matoi and Grey, however, with some small repetitive steps and setups in rom hacking, you can get around this.
First, you need to prep them up, have them ready to S support, but don't do it just yet, save your file and go to your romfs to do some editing.
Second, make a backup of your romfs, or just GameData.bin.lz as you'd edit GameData.bin.lz, located in the GameData folder of the romfs.
Drag into FEAT to decompress, then drag the .bin file into your hex editor of choice, you'd have to locate either Saizou or Tsubaki's character block.
Tsubaki's character block starts at address 0x1478, the very first byte in the character block tells the game whether this character is male, female, male MU or Female MU.
Tsubaki's value should be set to 00 (male), change this to 01 (female) and save the changes you have done to Tsubaki's character block, compress the bin file with BatchLZ77, rename it to GameData.bin.lz, replace it in the romfs and load up HANS (or NTR).
Next, have Tsubaki and Saizou marry, if done correctly, both Grey's and Matoi's paralogues should unlock as expected, because the game reads the marriage as Saizou(male) and Tsubaki(edited character block so now flagged as female), recruit both child characters, and save the game.
Note that you shouldn't leave Tsubaki flagged as female, because this will affect his classes and the game will force him to only be able to reclass to female options, so either simply restore your backup romfs/GameData.bin.lz, or go through the decompress/edit/compress process again to flag Tsubaki back as a male character.
This hasn't been tested yet, but there is no reason why this shouldn't work.