Different strokes for different folks. I personally could/would never do this. It isn't even fantasy material.
I can see if there is a casual sex kind of relationship with someone then it's party time but not with my wife. Nada. No way.
Whenever I read about guys who really want their wives fucked by a big dick while they watch I can't help but see it as using the woman as a prop in a guy fantasy where the bull makes the husband's fantasies come true. It's all about choreographing the fantasy for the husband's pleasure even if he says it is for the wife's pleasure.
For the bull it's just great fun.
I make a distinction between this and swingers who seem to just have a very different view of sex within a marriage with both partners screwing their respective brains out.
My ex-GF and I use to have threesomes all the time. She met a guy at her work who she really thought was hot. Before anything happened she would always talk about him so I pretty much knew she had the hots for him. One day while I was pulling a night shift we were talking on the phone. She was telling me how turned on she was. Out of the blue I mentioned she should call "him" and see if he'd like to fuck. I said it half jokingly but her response was all serious! She said "I'm sure he'd love to fuck me but how pissed would you be if I let him?". While sitting in my car on the phone I found myself with a HUGE hardon. I told her that the only condition is she had to let me eat her out when I got home while she told me all about it. That was the beginning of a VERY good relationship. Sometimes I'd come home to see his bare ass plowing her. There was just something so hot about watching another guys huge cock pound your GF's pussy.
I'm straight but I can tell you I do get turned on seeing another man's cock at full attention and servicing a well deserving woman!!!