Voltaire was a brilliant playwright and author who was beloved by French royal society, and Emilie was a young, intelligent socialite.
Emilie was married to the Marquis du Chatelet, but neither she nor Voltaire cared about what people thought - they went out and about together as a couple for the fifteen years until Emilie died, even living together in a house owned by her husband.
These two were not only attracted to each other physically, but even more so attracted to each other's superior intellect.