It IS about the kids if a couple chooses to have kids. Even if a couple discovers they they never should have married each other, they should design any new arrangements to make it as easy on the kids as possible.
It IS the case sometimes that remaining together may be harder on the kids than separating, but every effort should be made. If infidelity is a major issue, it 'can' be better for kids not to watch the game play out.

   I was personally acquainted a number of years ago with some people who had a very 'open' lifestyle with no jealousy and where the kids knew everyone and 'flowing relationships' were taken for granted. But each family was still defined, and none of the kids doubted who loved them and took care of them. Unusual, but they seemed to make it work. I do NOT recommend that just anyone try this.