Fates is really a mess, but we already knew that. Honestly in my opinion, marrying Hinoka, Takumi, Sakura, or Ryoma is not that bad. You didn't grow up with them and they are not related to you by blood at all.
Marrying Azura is incest, so yeah, that's fucked up, and the Camilla, Leo, Xander and Elise... well at least they aren't really related to you, but you still grew up thinking they were, so I dunno.
It might be legal, because no blood-relation, but it's still too weird and creepy. And about the incest with the second gen sheananigans... yeah, that was also possible in Awakening (and Genealogy/Thracia lol) depending on who you pair-up with who to make the babies, and honestly I want to think it was an accident. Like, they didn't think hard enough about it to realize it could happen. But at the same time, this is Fire Emblem, a series infamous for playing fast and loose with both pesudo-incest and actual incest, so maybe I shouldn't give them the benefit of the doubt.