FE games weren't so much focused on the "anime-ness" of their systems, if you even want to call it that. They were mostly focused on the story previously. If you go over some of the support conversations, there were some out there ones in the past too.
I believe that with hte current FE trend, a lot of these features are to bring back in a lot more of the "niche" crowd and cult popularity in Japan due to the absence of an FE game for so long. This translated ok over to here, thanks to great writing and translation, and created the whole shipping mindset and general anime tendencies we tend to associate with Awakening. In reality the game's features are true to Fire Emblem standards, even if I have some issues with in.
In Awakening, the time travel wasn't mindlessly tacked on, it was done in a pretty fair way, and the whole Children units thing, along with the more funny support conversations, created this mindset in a lot of new players about grinding not only to get strong and feel like an unstoppable machine, but also earn said supports with which to enjoy.
So with that in perspective, the changes made in Fates create a paradox. They wanted to keep child units in order to help keep that interested crowd in. However, with the way the story works and characters relate, it means that the romances with a lot of possible characters are moreso along the lines of "Adopted Family" rather than friends you grow with along the way. Given the apparent age and way some of these characters act in the Jap version, has obviously not translated so positively to America and other countries given the current viewpoint on incest and other relationship issues (like with the whole stupid drama around the one young girl who seems to have a greater interest in girls.). This doesn't apply as much to the Nohr side given that you discover you're not related by blood very early and by that point you've grown up as basically a stranger to this "family" so it makes more sense in that regard that romances are an option. Hoshido has less of an excuse, however.
My standpoint on it is that Nohr is fine. We're not directly related to anyone and we find this out early enough and since we didn't grow up with the Nohr "family," we're basically strangers to each other. In that regard its as stated above about them.
In reality, however, I don't think it matters. You can be disgusted by some of these things, but the reality of it is that its a video game. It's obviously not emulating reality, and this whole culture shock is more seen as a fun side thing. It's more a side thing for you to pair people up, as in the previous game, and the marriages an extra reward. Honestly it feels like marriages were tacked on because "They were popular in Awakening."
tl;dr: Regardless of your feelings, Fates is trying to keep a mechanic that was popular in Awakening. It does create some awkward issues, given the "political correctness" of America as opposed to Japan, where stuff like this mostly seen as a fun little fantasy diversion completely unrelated to reality.