From the book Sir Isaac Newton: Brilliant Mathematician and Scientist:
(by Natalie M. Rosinsky)
[Isaac Newton] spent almost all of 1689 in London [...] At that time, Newton began what was possibly the strongest personal relationship of his life. It was with a young Swiss mathematician named Nicolas Fatio de Duillier [...] While there is no proof that either man was homosexual, their letters suggest that their affection for one another was romantic.