This is a bit off topic, but I just read the book you referenced in your first post, Sperm Wars by Robin Baker. Parts of it were very disturbing to me, but I found all of it to be enlightening and interesting.
I'm a little confused at your interpretation as it applies to this topic though. A big part of the book stresses that men and women often have different things to gain and lose when they have sex (as it relates to their chances to reproduce and have grandchildren). From this perspective, its hard to see how this fantasy could have positive reproductive ramifications for the man who wants to see his long term partner have sex with another man. He would be initiating a sperm war that he might very well lose, and thus reducing his own chance of fathering another child with his partner. If he does
lose the sperm war and thus his partner is impregnated with another man's baby, its hard to see any upside for him, reproductively speaking.
Do you have another conclusion? There must be some
evolutionary upside to this that I haven't thought of. Otherwise this fantasy wouldn't be common to you, me, and so many other men.