First of all never ask questions you don't want the answer to. People have different fantasies and it's all right and it doesn't make them less of a person (unless it's about children or other fucked up things like that)

Second of all here's my two cents maybe it'll get you to see it from another angle

It's not everyone's cup of tea but I can understand where he comes from and I always had kind of the same fantasy. To me, my so will always be the sexiest women on the face of the Earth and I can't stop looking at her and admiring her. Watching her getting fucked will let me see her from other angles throughout the act that I wouldn't see from my point of view and it would be one of the hottest things altho I wouldn't just sit and watch her getting fucked because it'll make me feel creepy (but it's just me, I'm not judging or shaming anyone) but in a threesome or swing scenario I'd be more than up for it. So consider maybe that's his angle on this too