Found my answer! Very hot. Having been on either side of this equation, it is a very dominat feeling to fuck a man in the ass. I find it very hot when I've taken some men to reach between their legs and feel how hard my cock up their ass makes theirs.
My hubs is a good sport about all of it. He knows what turns me on and what I enjoy and goes along with pretty much anything. But, as you point out, when he is getting his ass filled, I see how hard his cock is, and I know he is enjoying it too.
I must admit, at first, when guys would want my husband in panties or would fuck his ass, I had tinges of jealousy and doubt. I could not figure out why a man would pay attention to my husband when I was the one they were with. I got close to one hot guy who seemed to enjoy fucking my husband, and I asked him one time if he was using me to get to my husband. He just laughed and told me the same thing you stated: It gave him a huge sense of dominance and power to fuck a man's ass in front of his wife or require him to wear panties. That really brought it into perspective for me; it had nothing to do with me but was about the dynamic between the two men struggling for dominance. Men love structure, and they love to be the ALPHA. Men are pack creatures; just like a den of wolves, one has to be the Alpha.
I now understand if I want the guy to have a raging hard-on and to fuck me hard as I enjoy, he has to feel that sense of domination and control of the situation. This is why I will often have my husband wearing panties when we meet, and I will offer my husband's ass. A bull fucking my husband has nothing to do with the bull's sexuality. It is the age-old struggle between men to establish a dominance hierarchy. As long as men produce testosterone, this will always be the course. My husband agrees and says that when we meet a man, and he is wearing panties, it already puts him in a submissive mindset. Having another man fuck him in his ass cements the deal.
After all, how could he feel dominant when he is presented before a bull like this?