This story isn't about Gero continuing the bloodline for his sister's sake
I can see where this reading comes from, since Gero's sister is what kicked off the whole marriage hunt in the first place. But I think the story has moved in a direction where that premise doesn't hold the way it used to, so I wanted to lay out how I see it.
- How I read the series
I like breaking down the dialogue and panels in this series, and I read it as two stories layered together: Gero's search for someone he genuinely wants to spend his life with, and his reckoning with the oppressive structure of his own family.
- How I see other readings
Given where the story is heading, I think Kinosaki is the most likely partner right now. But it isn't finished, and the author can still take it in a different direction, so I find the readings of people who support other heroines genuinely interesting. In some places their readings might even fit better.
- Why I disagree with "if Gero doesn't have an heir, his sister will be forced to marry a man, so it doesn't fit the theme"
First, the series separated Gero's marriage from the heir requirement from chapter 1. When Mei asks whether he really wants to marry just for the sake of the family line, Gero answers that he wants to meet someone he truly loves and live an ordinary, happy life with them, and Mei takes him on as his advisor on exactly those terms. His sister started the marriage hunt, but what he wants from it was never an heir.
Second, >!in the latest chapters, "marrying to continue the bloodline" has become the very thing he sets out to break. Gero names his grandmother as the last and biggest shackle he has to overcome, and declares he's going to take down his own family. It's natural to read that as including the forced marriage hunt and the family's broken rules. Once that system is gone, there's no one left to force his sister to marry anyone, so it makes more sense to see her problem being resolved as part of that process.!<
To be clear, I'm not saying Gero can't end up with one of the heroines or have kids. My point is that the story is moving away from the idea that he has to produce an heir for the clan. Whoever he ends up with, it should come from his own choice rather than an obligation to his family, and that seems to be exactly what the story is building toward.