its a kink
Indeed. But that changes the body-type in a somewhat different way though. (A pregnant belly will be more firm, for example.) And not everyone will be put off by the idea of a child being in there. It does mean you need to tread the lady and especially her belly with more care, for sure.
Lots of adult games featuring pregnancy treat the actual 'child' portion of the pregnancy by ignoring it. Some might even go as far as featuring some kind of 'fake pregnancy' that canonically doesn't involve a kid.
That said, a certain amount of (perceived/cultural) wrongness about it might also appeal to some. Certainly its not a crime in the real world, for a pregnant woman to have sex with her husband/lover. So any disapproval would be a cultural one, not a legal one. And something that is a little bit wrong can be exciting too.
Alternatively it can also go a pretty wholesome direction. In a 'ok, now that this happened, let me take care of you', sort of way. Where the MC can step up as care giver, life improver and reliable strong figure as the future dad.
It could be potentially exponentially more work to add such a feature in, versus how many people want it (and some people would want it disabled, which means that whether it was enabled or disabled would need to be accounted for at the end of every relevant scene, which to be honest would probably not be too hard to do but would likely take a long time. Then based on how the model works, they would need to change based on whether or not they're pregnant, and they would also need to add that for every existing model and outfit, which sounds like a lot of work and sounds like it would take exponentially more time for each outfit that existed in the game) And then if there are any errors that arrise between scripts and rendering, those would then need to be fixed, and every relevant animation would need to be checked to ensure it isn't broken by the model change. Every relevant scripted scene may also need to logically change with potentially different models exclusively for that scene. I wonder whether the dev would do it based on that alone.
It could made into a bullet point list, but I think the question would come down to how many people would really want it for whether the development time for it is worth it, and not better spent on another area. There's also the thing where it's potentially going through this whole process again each and every time a new romance character is added. And whether most people would turn it off, why even implement it?
My point is basically, it would likely depend on how many people would use the feature, it is worth the technical debt it may incur? By that I mean, potentially making it more difficult to add new features without a potential rewrite of the whole system. I don't know, but I could likely see the dev not doing it because of that.