Probably because he has no interest in other women except for me. He tells me that he gets all of his sexual pleasure by my pleasure. That's why I think the fantasy is so hot and I have a hard time getting it out of my head.
I too have that fantasy about my girlfriend. The difference is that I don't think I would want to be there when she is with another guy. I'd prefer for her to go to his place and have sex with him there then she could come back home to tell me about it and we would have a great time. The commonality I have with your husband is that I love to think of my girlfriend having great orgasmic pleasure with another man. When I have this fantasy I always see her face contorted in the throes of an orgasm. If I fantasize that I am there while it happens, then I imagine her telling me how good his cock is and how much she loves the way he is fucking her. For me, it is all about her pleasure. I have googled about this 'kink' and as you probably know by now, it is fairly common. Not many men would admit this fantasy to other guys so I think it is somewhat hidden away amongst us.
Here is one thing I want to mention. I had a girlfriend some years ago with whom I discussed this fantasy. She was turned on by it but never acted on it. I figured she was capable of doing so, but I never pushed her about it. We had been having problems with the relationship (this fact is integral to the story) and one night had a seldom accompanied male friend with us out for a night at a local bar. I was an asshole that night, running around flirting and not paying much attention to my GF due to our relationship issues. The guy (who was around us very rarely) easily spotted the opportunity in the situation. When I passed out at home that night my GF woke me and asked was it ok for him and her to go get some late night breakfast. I slept thru the night and awoke to have her tell me later in the day that they went to listen to music in the car in a secluded spot after they ate. Long story short, she ended up sucking him off in the car. As she confessed to me I was very powerfully turned on. I got so incredibly aroused that I asked her to tell me all the details and I had her suck me off too. Thanks goodness she was feeling guilty and did my bidding because she was willing to do it again a short time later. I was so aroused! But...in time it worked on me and it ended up being a big factor in our ultimate break up. One thing that ate at me was that she didn't discuss it with me ahead of time....I felt she betrayed me...even though it had been part of our fantasy repertoire. The other factor was that our relationship was already well on the rocks. This particular scenario didn't involve the idea of her getting a great deal of pleasure like my present day fantasy but it was still very, very hot. She did enjoy performing oral sex and she told me she was wet and very aroused but she couldn't seem to let him fuck her. I enjoyed the picture of it in my head and even fantasized about her fingering herself to orgasm while she did it. Even though that didn't happen I would have been even more excited if she had confessed to doing that. So...it seems consistent that for me and your husband, it is pleasurable for us to imagine our lady's pleasure. Why? I don't know. Do we feel inadequate and find pleasure in knowing you are pleased with another guy? I am mixed up about that part. One thing I do know is I enjoy the fantasy and reality of sharing wife/gf. Another aspect of this is that I like to think of her being a 'dirty girl'. There is something very arousing about that, her having a secret guy that she goes to to be 'bad' and have 'secret' sex. It is like a drug for me...so intense a fantasy. So, that is my tale.
I would actually gladly be willing for my present day girlfriend to have a guy to please her, and if she was to come to ask me if she could suck a guy off, or have sex with him, I don't think I could deny her. However there are two things that I would need reassurance about. 1. I would have to feel comfortable that she wouldn't be stolen away by this guy, that it's only about pleasure and fun and not falling in love. 2. I wouldn't want him to be someone close to us. I wouldn't want it to be a good friend and would prefer that it be someone she has 'secretly' on the side. He would think it was hidden from me...she wouldn't let him know that I know.
So, what am I saying? I wouldn't presume to advise you to actually do this. It is something you two need to discuss with seriousness. Also, I would say to set up ground rules about it that you both agree to. If there appears any issue with what is going on for either of you, stop it and reassure. I am sure there are people who make this sort of thing work for both their pleasure, but there are also many who have probably had things go badly from it. Make up your minds together. As for me, I'd love to see if it would work, But my GF isn't willing to do anything more than talk about it for my arousal. That works too! Good luck to you both.