The article doesn't touch upon it - but IMO, the gender of that "best friend" matters a lot. Most people would probably be just fine with their spouse having another best friend of the same gender, but definitely not of the opposite.
It seems to me that people to whom this makes a difference are both extremely insecure in their partner’s loyalty and extremely secure in their partner’s exclusive heterosexuality.
Take away the former and the concern disappears, take away the latter and the concern remains without the gender disparity.
Better to spend quality time with your spouse and re-enforce the bond the two of you have for each other.