I'm not going to go more into this, as I haven't read the books and so I don't know all the context. To me it sounds nonsensical, but I do not have all the information.
It's all in my OP post, and the convo between Geralt and Emhyr there.
For a short version: Emhyr's father was overthrown and killed while Emhyr was cursed to become a monster for most of the day besides after midnight until dawn. Few people knew he was cursed and he was presumed dead by the usurper. The curse made him into a deformed fury but with his intellect intact.
In truth Emhyr survived and was hidden away by a very loyal supporter while he sought to free himself from the curse. He discovered one but it required him to win the love of Pavetta. He posed as a minor noble and was able to win Pavette'a heart and hand in marriage, which was required for the curse to be broken, with Geralt's help. Geralt asked for their first born child as a reward, provided it was a boy.
After marrying Pavetta he discovered the truth about the prophecy from Vilgefortz and planned a way to return to Nilfgaard without letting everyone know that Emhyr was Duny, because being a mosnter, a freak, marrying a foreign born, would all be heavily frowned upon in Nilfgaard and Emhyr was to lead a revolution, a revolution gaining traction with the help of Emhyr's loyal supporter.
So he planned to fake Dunny's death with Vilgefortz's help while taking Ciri and Pavetta with him while on a boat back to Cintra from Skellige ( Skellige and Cintra had very good relations at the time ). Pavetta ruined this plan by suspecting something affot, she died in an accident while Emhyr barely survived.
The prophecy is that Ciri's child would rule the whole world and save it from the White Frost. To Emhyr it seemed crucial that said child become the ruler of the Empire as soon as possible. There's either the idea that he should marry Ciri in order to make that happen since then he could pacify Cintra, save the world and give Nilfgaard a true blood Emperor, that would be accepted...grudgingly.
Then there's the option of not marrying Ciri. Which as I pointed out would cause a whole lot of issues since in order to have her child to become heir and be accepted by the nobility as such it MUST be of a Nilfgaardian lineage. Nilfgaard may not have discrimination against non-humans on a normal societal level, but it comes to the ruling class they are just as elitist as the north is.
So Emhyr would have to reveal he is Ciri's father, which means revealing he was a monster and that he married a foreign born woman years before he became Emperor. If key members of the nobility are already plotting against him then the fact of him being a monster becoming public knowledge would lead to open rebellion against him and no Nilfgaardian would easily accept the grandson of a monster on the throne and it would be a REALLY hard sell to any noble from Nilfgaard to marry Ciri in order to give her legitimacy. Voorhis seems up to the task, or so the game suggests since he's a loyalist. Either way she would still be in an arranged marriage.
Are there any benefits whatsoever to Emhyr not marrying Ciri from a political level and keeping his past as Duny a secret? I see none. At least none in the scenario where he wants to save the world from the White Frost by placing Ciri's son on the throne of the Empire.