It's common among married couples too. Though they are more common in marriages than a non-committed relationship such as gf/bf. This is due to what goes through your head in the long-term effects of marriage.
Which is, most married couples are afraid of one thing: alone. Being alone is the type of thing that scares most couples. Now which ever partner feels threaten by this they're more open to have dramatic changes. If that partner tend to come out as insecure, or feelings of compersion seeing partner with another. Then they may end up in one of the starting phase of cuckold or cuckquean depending on who is getting cucked.
Cuckold is more common on men than women because in my opinion men tend to watch porn more than our opposite. Some of it had to do with how watching a guy fucking a girl on the screen. Beat your meat to that and your brain will start to rewired itself. If you do it a lot you'd subconsciously start to think it's natural if it happens irl. Beating your meat while watching two people having sex right in front of you. It's like watching porn but irl. How cool is that. Joke. If you mix porn with your SO than you are cucked.
How do couples find others to bang there wife/gf?
r/willyoufuckmywife, r/cuckold, FetLife.com, MF4M Craigslist, tinder, OkCupid, and friends (optional)