Not sure if it's common... But I do have a few couples whom are my friends that behave in such way... Similar to your colleague/friend...
1st Case:
He and She have been together for more than 8yrs, before settling down. Before settling down, they both had a 'weird' agreement. Why weird? One of their agreement said that He and She can both have affair outside, but DISCREET & SAFETY is a must. Also, they CANNOT bring home their partner/affair...
They are both happily married till now which is about 10yrs of marriage already and they enjoy childless marriage...
2nd Case:
He and She have been married for about 9yrs. They both had 2 kids of age 5 and 3 respectively... 4yrs ago after the 1st child was born, He had an affair... However, She decide to forgive him and continue their marriage life... Reason:
1) He comes home daily after work to accompany the family for dinner, gathering etc....
2) He still support the family by paying for all necessary expenses that may incurred
3) Sat & Sun & PH will confirm be with family as well.
4) He still have his affair outside but only during working time
