I feel that promiscuity is fine for any people who are in a relationship that is logical (emotionally distant? emotionally inclusive?) enough to handle it. Promiscuity is not good for any relationship when it is not an agreed upon behavior for all individuals within the "promiscuous" relationship.
A link showing the most highly regarded reasons for the fall of the Roman Empire does not show "promiscuity" as a reason for the decline: http://www.history.com/news/history-lists/8-reasons-why-rome-fell

Personally, I think it would be very interesting if it was legalized and then someone came up with the equivalent of "Uber" for the sex industry.