So here’s my additional two cents (if it’s even really that much) on the topic, in the form of mostly disjointed brainstorming:
Summing up your post (thanks again for bothering with me! ❤️), we do not really have any detailed sources about how and when Napoleon really considered himself sterile or feared that he might be. These are merely conjectures by either contemporaries (though I would not know of one) or by later historians (who of course all had and have different agendas when they interpreted Napoleon’s rule and character).
Which kinda begs the question if we really need to speculate on a particular additional motivation for Napoleon’s cheating on Josephine, considering that he came from a patriarchal society that largely felt like the vow of marital fidelity simply did not apply to the husband. Unless we absolutely want to make sure to give Napoleon a motivation for his cheating that somehow seems more „honourable“ or excusable (i.e., trying to figure out if he was sterile or not).
Sounds a lot like Frédéric Masson.
Speaking of Masson, I’ve looked into Branda’s biography of Josephine, and Masson’s „Napoléon et sa famille“ seems to be the source for some of Josephine’s physical symptoms he describes, for everything after 1799 apparently the only source. I wish we were given some more information on where Masson in turn had his information from (but I so often wish for that).
According to Branda/probably Masson, Josephine did not have a menstrual period from 1796 on, with a brief episode of hope around 1800 when Corvisart became Josephine’s physician and she briefly had bleedings (that, according to Branda, however only were a consequence of the inflammation she had suffered in 1796 and which had left Josephine infertile).
While I assume that at the time indeed most people (patriarchal society and all) still believed that a baby came from solely the male seed, with the mother being nothing but the vessel to grow it in, the connection between menstrual cycle and female fertility was well-known. (We can tell from the Russian argumentation when Napoleon wanted to marry tsar Alexander’s sister in late 1809 and they responded that Anne had not yet had her first menstruation.)
So if it’s true that Josephine did not have a menstruation anymore after 1796, why would Napoleon have feared that he himself was the problem? In this case, it was obvious where the problem was, and that Napoleon could be as capable of fathering children as he wanted; his „vessel“ was broken.
And I do believe that this is the case and that Napoleon never exptected to have or planned on having children of his own at the beginning of his political career. The question of heredity comes up almost immediately once Napoleon had secured his power after Brumaire, and Napoleon always envisioned that he would adopt one of his nephews. Which finally brings us to Napoleon’s brothers.
There were two Bonaparte brothers who fathered sons during the Consulate: Louis (whose oldest son was born in 1802) and Lucien (whose oldest son was born in 1803, but under rather dubious circumstances according to Citoyen First Consul). Let’s not completely forget Napoleon’s actual oldest nephews from his sisters, Pauline’s son Dermide Leclerc, Elisa’s son Félix (who however died in 1799) and most of all Caroline’s son Achille Murat. Of course Napoleon did not truly count those, but their parents might have held a different view and may still have hoped for one of them to be the chosen heir.
Joseph and Lucien at the time were in almost open opposition to Napoleon. While Napoleon was already planning a dynasty (based on himself), Joseph and Lucien mostly wanted a share of the power for themselves. They cared a lot less for anything that would come in the future, they just wanted to cash in now. Joseph had experienced first hand during the peace negotiations, when he had been allowed to play the special envoy, how little leeway somebody working under Napoleon’s command would receive, and Lucien had had similar experiences during his term as minister of the Interior and as ambassador to Spain. Joseph refused to become president of the Cisalpine/Italian Republic, and Lucien went a step further and exiled himself to the Papal States.
Already at this time Joseph vehemently opposed Napoleon’s adoption plans (and he managed to put pressure on Louis in that regard) because an adopted heir would interfere with his own ambitions to succeed his brother. The topic came up again before the coronation and led to a vicious, vicious dispute between the two oldest Bonaparte brothers. According to Joseph, if Josephine was crowned, this would put Louis’s children before his own two daughters, whose mother was a mere commoner, while Louis’s sons through their mother Hortense could claim descendence from an empress.
In the end, Napoleon was forced to accept a sort of compromise: Joseph was declared first Prince of France, meaning that he would succeed Napoleon. But the Constitution to Joseph’s great frustration also allowed the emperor to adopt a son of one of his brothers. Which, with Lucien exiled, could only refer to Louis. So the threat of being surpassed by somebody from the next generation was still there.
Where am I going with this? I’m not sure myself 😋. Logically, the Bonaparte brothers should have been happy to have Napoleon stay with his infertile wife because, as long as Napoleon did not adopt one of his nephews, this would guarantee their succession. However, plans for or at least rumours about a divorce were there already since before the Empire (1803 at the latest). As the reason for the divorce always was the idea that Napoleon should have children of his own, apparently nobody imagined that Napoleon could not father any but that Josephine was indeed infertile. Joseph in particular, as the oldest brother and nominal head of the family, would have welcomed an heir who was only born after a divorce as, in the case of Napoleon’s death - which apparently everybody expected at every moment -, he still would have succeeded his brother as regent for an heir who in all likelyhood still was a minor.
As to Napoleon only "divorcing" Josephine in 1809, I would argue that Fouché had already talked to her in 1807, and according to Napoleon, because he wanted to comply with Napoleon's wishes without having officially received the order to do so. There's also Eugène's letter from the same year, telling his mother that he had heard rumors of the impending divorce from several sides, Munich among them. So Napoleon may have decided about it a lot earlier (personally, I'm convinced the topic came up at Tilsit with Alexander). The fact he does not immediately do it may have had different reasons altogether (like the coup at Bayonne where he felt he needed Josephine by his side etc.)
