I know how you must feel!
Ive been trying for 3-4 years to get Mrs C to at least entertain the idea of taking anorher cock with no luck.
The odd thing is that during foreplay I tell her my "dreams" about her with other men in various situations and positions, she gets really horney and super wet, she cums so hard. I can tell that the idea excites her but outside of our lovemaking, she totally refuses the idea of cuckolding. She even gets angry at me for persisting with my thoughts.
One time I arranged a sensual massage where she was virtually naked with the guy, in the hope that she would be so turned on that she would "give in".... no such luck. My cock was so hard watching him touch her all over. He was rubbing her ass and pussy but nothing further happened.
The best we do now is show her off in public, usually in short skirts or sheer swimsuits. I have to be content with guys fucking her with their eyes.