I'm not sure if this is against the forum rules - but it's a perfectly valid area of academic interest so I'll go on ahead.
I guess we all know about Voltaire's flirtatious correspondence with Frederick the Great, where the two speak to each other almost like lovers, to the point - and I've read some quotes from these letters - that it seems to go too far to be in jest. I've also read a love letter of sorts he sent to Lord Hervey (it's made clear it was not addressed to Lady Hervey). The journal article "Unhappy Voltaire, or 'I'll Never Get Over it as Long as I Live' delves into his sexuality and views on gender which really makes me wonder if he was bisexual. While people have said he was critical of homosexuality, I'm no expert on history but from what I've read he's surprisingly tolerant of it for the 18th century - he implies for example that homosexuality is a natural inclination and not a choice.
If only I could find more resources than second hand interpretations of most of his letters. *sigh*