I've never worked at any company that used them or heard about them. Instead we take yearly training saying that even flirting with the idea is a cause for immediate termination.

Do you have stats on how common these are? Is it more popular in some non US country maybe?

Do you suppose Jensen had a love contract with his subordinate, when the breaking of the news of their extramarital relationship led to a 20% drop in stock and the removal of Jensen as CEO?