Having read the Zinoviev-Lenin correspondence the letters strongly imply a more-than-only-political relationship between the two Revolutionaries.
But I know Lenin had several mistresses like Inessa Armand beside his wife.
This would mean he was bisexual.
Or are the Zinoviev letters fake? ( I can only find them in Russian, but I guess that's a taboo topic there anyway).
I don't think it's a taboo, in theory something like that could be researched and discussed but it'd be a hot topic nonetheless.
Anyway, these letters are absolutely fake. You don't have to be a historian to get that; if Russian is your first language, it's obvious. Writing style, rising level of craziness and so on. Actually, I don't think that 'fake' is appropriate word here, it's more like a prank — unknown author of this publication didn't try to fool anyone, it's more like a joke for him. Another such project (and much more popular and smart) was 'Lenin is a mushroom': in 1990s on Russian TV was shown some mocumentary where 'scientists' were talking about new discovery that Lenin was not a man, but a human-like mushroom.
It's the same. The author is completely unknown, there are no any links to the sources in the original text (just 'I've found these letters in some archives'), there are no photocopies, it wasn't scientific paper et cetera.