As a firm believer in the tenet that the human animal, such as we are, is not - nor was ever - geneteically predisposed to be monogamous, I am nevertheless a proponent of responsible non-monogamy as opposed to gratuitous, anonymous sexual encounters. While each may have its place, the former allows for the development of stronger and more vibrant relationship between partners who are on the same or similar pages when they embark on this journey.
As the host of a group that currently has 3000 sexually explorative member couples, I have been witness to most everything, bad and good, that might present itself in situations of this sort. I have seen couples blossom by coming out and opening themselves up to sexual exploration with others. Generally, these are couples with already strong bonds that are looking to expand their horizons by including others in their sex play - be they one time encounters or ongoing intimate relationships.