How are they going to prove it? Show their marriage certificate and drivers license? I doubt you're ever going to get proof cuckolding on porn. Though now that you mention it there should a niche for reality styled porn like this, you know like Bang Bus.
Brazzers just started a start like that recently: http://www.realwifestories.com/ Most of the 10 or so scenes they launched the site with had the "husband" watching, though I'm not completely sure they're going to keep doing it at that rate in the future.
I saw a scene with Chelsie Rae where her real life SO watches her get fucked by multiple guys, at least for the first part of the scene, I don't remember if it was for the whole scene. I'm sorry I can't remember which flick it was from, but if it helps to locate it, I'm 75% sure it was a Chico Wang flick (not 100%, not 90%, but better than 50% sure), and like I mentioned, she gets fucked by at least 3 guys, and I'm also 75% sure they were all white guys. That should help narrow it down.
starsky5000,not sure how real this is but there is a series called Wife Watchers Confidential from Rude Britannia and features Johnny Rebel,this is some box covers and reviewsLink 1,Link 2,and Link 3 if it is what you are looking for then maybe a trip to "The Marketplace" on the forums to see if anyone is selling any might be an option for you.
Well, there's Candy Apples Busted at 742, where Candy Apples attempts to have sex with 1000 people with her fiancé being person 1000. Not sure how authentic it is, but well, she does manage to have sex with 742 people (!).