Because, for every X, there is a philosophy of X, it should come as no surprise that a well-known philosopher has written a book on this subject! I refer you to Richard Taylor's Having Love Affairs (Buffalo: Prometheus Books, 1982), isbn 0-87975-186-X, http://www.amazon.com/Having-Love-Affairs-Richard-Taylor/dp/087975186X
Is adultery really immoral? The act itself is mostly legal, so why can't it be mostly moral? I'm a male bachelor, so I can only argue from my point of view. Adultery is a simple biological urge that manifests itself onto two persons, one or both of whom are married. Marriage today is becoming more and more a simple legal contract, routinely terminated and routinely redefined by judges and plebiscites. The ease with which marriages can be terminated either on paper or in practice is just a reflection of the fact that people often change in their feelings towards one another--love fades within marriage and sometimes erupts outside marriage. Making it with a married woman can be very thrilling and the same woman would not be equally exciting if she were single; the supposedly unavailable is always more desirable than the easily attainable. Married women accept advances because their husbands can no longer give them excitement, romance or adventure, so why not a net utilitarian gain for two people, and no change for the unknowing cuckold?