I know older couples who are all but separated but didn't do it officially but I think its different for such a public dude as Bill Gates. Like even if his wife was okay with him moving on and seeing other women he would get roasted if he were out in public hugging up with another woman.
I would be the only reason they are going public and making it official is that one of them have plans to move on with their next lover.
********** "Everyone has a plan until you punch them in the face. Then they don't have a plan anymore." (c) Mike Tyson