During a former relationship, I would masturbate to thoughts of my girlfriend obtaining pleasure from another woman, and quite regularly, other guys. She was submissive and wished to please me above all else, so the idea of her becoming aroused to the point that she would seek
her own pleasure first and foremost, without consideration for anything but sexual desire, was an immense turn-on.
A few fantasies:
-Her becoming hot and bothered with a co-worker and fooling around on the nearest desk, coming home with soaked panties and allowing me to go down on her
-Deciding to wear a skimpy skirt to the bar and having some stud nail her in the backseat of his car, possibly venturing back to his pad for a raunchy gang bang
-Using a webcam with a friend while drinking a bit too much wine, finding her furiously masturbating and flirting away, thighs spread wide in her chair as I peek into the room
I can confirm that jealousy is a competing emotion when considering these fantasies as actual possibilities, and that always prevented me from genuinely proposing what your husband has, which is offering your body to another man. With my girlfriend, she would often tease me about her naughty activities, how she hadn't been able to resist someone, or that she was horny enough to consider a fling. We both knew it was just playful banter, but the fantasizing was incredibly enjoyable.
You should be proud of your husband for mustering enough courage to openly admit his fantasies, but I urge you to discuss it with him and discover if any jealousy lingers in his mind, regardless of how fucking hot he considers your arse raised sky high with a thick cock balls deep inside of you.