To actually understand this quote, it's important to understand that Proudhon drew a very important line betweeen property and ownership. Property was something that you owned on paper, and derived profit from somebody else using, like a landlord being the proprietor of a tenement or a capitalist of a factory. Ownership, on the other hand, referred to something that you used to make a living or from day to day, like a farmer owns his plow. Ownership was okey-dokey, property was, of course, theft.
Proudhon's typically French metaphor was this: the wife is the property of the husband, but two lovers own each other.