Yeah but still it's pregnancy nothing really to it..
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.