Clarisse Rey was out of her element socially, and probably felt like an outsider all the time. She didn't receive any warmth from her husband or his family. Hence, she accidentally found another outlet for herself in a time period when homosexuality was not acceptable to society.
This was a very warm woman who had no outlet for that warmth. She couldn't even enjoy much closeness with her children; raising them was thought to be the task of the servants (e.g governess). Seeing them just once a day was not enough for her.
The family was probably aware of her feelings since the mother-in-law had her followed and photographed. For them, in that time an culture, her activity was sinful, and to be hidden. She was treated by a pariah.