It's just a kink. Kind of like sadism and masochism. A logical mind says a woman in black leather stomping on you with spiked heels leaving little bloody divots in your back and whipping you with a horse crop before fucking you in the ass with a 12 inch dildo is something that could only be enjoyed by someone who isn't in their right mind, but it is just a kink. Just as people can enjoy inflicting or receiving pain some individuals experience pleasure from observing the people they care about enjoying sexual pleasure with other people. They even find the feelings of jealousy to be pleasurable. I haven't experienced this myself but I did have a girlfriend who professed it and I observed her enjoyment of it. It is called compersion. If they care about both the people who are having sex, it is double the pleasure. So don't be so judgemental just because it isn't something that floats your boat.
I think that's a good summary, also I'd add there could be other motivations behind it - as one of my favorite games "nothing is forever" narrates in dialogue during one scene -
It could just be a strong voyeurism fetish; most people get turned on from watching people having sex, and if the brain connection of "I know those people, that's my girlfriend/wife" is blocked out, they can purely enjoy the visual aspect.
I know for me this is more the case in enjoying these kinds of genres than any jealousy or humiliation aspect - it's as simple as "I like porn/watching people have sex" + "I think my gf is really hot" = "It's hot watching my girlfriend do real life porn in front of me". The stuntcock could be me or someone else, though generally its difficult to get the same visual appeal from first person (idc what anyone says pov porn is TRAAAASH) -but it could certainly be only myself and a mirror etc..
Unrelated - dev is gunning for a 1 star review in by book with this kind of business: