Listen, you can make many points in universe as to why Ryn could want/get a pussy.
"Because she loves you" or "Because she is your slave" are all valid. But, at the end of the day, the path PC chooses are all coralated with what writer wants. And Ryn, and the champion for that matter, likes the way she is as she is. There is nothing wrong with wanting something from a character -- I would like if we could turn Agni and Brienne into guys for example -- but we should not act that a NPC should fit into every conceivable box. Ryn is supposed to appeal to guys and gals that like subby, potentially romantic, dickgirls and I we should respect that.
I agree with you that, yes ultimately what happens in the story and the restrictions on the players choices is entirely up to the writers/devs. The point I'm trying to make is that this is bad writing, because it is clearly breaking many peoples suspension of disbelief.
I'm not trying to make an arguement for what I want in universe. For example many people want Bakugo and Deku to be gay for each other. Despite it being clear that Deku likes Uraraka. And its set up by the author that Deku is straight. That's not what I'm arguing for. I'm not trying to pull some rule 63 here with Ryn. As, i've said already Ryn keeping their dick is something I actually agree with Savin on, from a writing stand point. Especially when going down the confidence path. (I would rather have a pure girl Ryn, but narratively speaking Ryn staying as they are makes more sense.)
My issue with the state of the story is more akin to the problems people had with Fawkes in Fallout 3. Why is the game/writer making a big deal about us sending in Fawkes (a character who is immune to radiation) into the water purifier. The writers want us to die a heroic death saving the capital waste, but we don't have to as there are multiple ways to solve the problem. This is a writing issue. As what we know about the world is not making logical sense.
Why are we getting Elthara pregnant? She's not a part of the royal family. Any children she has, would have less claim to the throne than Alissa's imps. Nothing about this makes sense when considering the characters goals. Ryn is set up as wanting to protect the Boreal Elves/Winter and falling in love or becoming the champs personal fuck toy, depending which path you're going down. This story breaks with what has been established by the character. This is Jake Skywalker all over again.
PS There are ways for Sav and Co to make this storyline work, Such as maybe Ryn needs a pure blooded Boreal Elf (oh wait we can become that). This issue is completely manufactured, and can be resolved just as quickly as it was added.