Tremendous insight and advice on here so far from James and Quath. My take is your wife may indeed be poly, if she ends up in a loving intimate relationship with this TK, if it's just a FWB deal, then she's just swinging......you may indeed be great at compersion, or you may just have a fetish. TK is not poly, as he is cheating on his wife.
Libido wanes for a variety of reasons, physical (like menopause/andropause), emotional (pick a reason!)......and of course for painful reasons, like abuse, neglect, etc......I wonder what TK's reason is?