Shifting desires seems pretty common. Here's an older thread addressing it and the NRE component. http://www.polyamory.com/forum/showthread.php?t=13998
I know women ( not many ) but a couple who like when their husbands to visit an occasional strip club ....getting them all charged up....and then being the recipient of that energy. This is the same concept except there is a sexual component beyond the visual. Do you think your husband or bf has done this with you....as either the fluffer or the person who is satisfying that impulse? do you care What stimulated his/ their mood ? No ...right
To the last part of your question I've seen several threads in which the primary couple lose interest sexually in one another. Pm if you want a list of user names to track down those threads.