> but if one of that couple got a promotion from the other then every other person on the team pretty much has grounds for a law suit.

Extremely rare (winning such a lawsuit). Prior to these kinds of rules becoming the norm, how often did anyone win such a lawsuit? It's a rounding error close to 0.

There are plenty of publicized cases where one party openly stated she got promotions because of her affair with a senior. No lawsuits were even filed in the ones I know.

I'm in favor of such rules precisely because it's hard to win such a suit.