What is the notion between do not mix buisness with pleasure meaning do have sex with coworkers? Isn't that little played out?

It's because more often than not these things don't work out longterm, one partner is hurt and it makes the workplace awkward for everybody. On the rare occasion that a workplace romance lasts it often results in different kinds of favoritism that also cause office friction. This is why most employers have strict rules against it. 

No, it's entirely relevant and will always be. It's plain commonsense for reasons similar to the old adage, "Don't shït where you eat."

Most casual sex relationships end up in a disaster when the two people involved are not on the same page—and mostly, they aren’t. Because the important part we miss out in this whole excitement of something new and wild is that if you both were on the same page, you probably would be in a relationship or just really good friends.