If they'll go back on prestablished characterization - that those characters are supposed to be family to Corrin - then yes, I'd say with absolute certainty that they let marriage options prioritize over the story themes and characterization.
That doesn't make sense. That's exactly what makes it fan service.
Unless you're operating under an entirely different definition of fanservice than I am, which I'm really beginning to think you are.
According to Tv Tropes (aka the most reliable source in the history of the universe), fanservice is:
"Characters in skimpy or skin-tight clothing, or none at all, to attract or "reward" viewers. This is traditionally associated with female characters and most common there, but fanservice of male characters
isn't unheard of."
By that definition, the marriage system in FE isn't fanservice at all. Is Camilla's armor fanservice? yes. She is wearing clothing designed to attract male viewers. Is being able to marry any character of the opposite gender fanservice? no. Many of them are wearing perfectly reasonable clothes, and none of them do anything like change outfits into something sexy for you - if the character (i.e. Camilla) was already fanservice, then they are still fanservice-y when you marry them. But characters like Hana, Selena, Beruka, etc (who are not fanservicey, IMO), do NOT suddenly become fanservice because you can marry them. The clothes they wear are the same, regardless of if they are your spouse or not. (It can be argued that I'm being pedantic about the fact that fanservice must involve clothing or the lack there of, but first of all the "sexy clothes" is a very large part of the definition of fanservice in my opinion, and second of all, even disregarding clothes, "sex appeal" for any character does not increase just because you can marry them. It's not like the people who were going to fantasize to Camilla are going to stop just because she's the main character's sister. They would still do it even if you couldn't S-support her.)
Tl;dr: Marriage =/= fanservice. Sexy clothes = fanservice.