Just good friends

I read "Just good friends".
It is a story of two couples. Stephany and Max go to Italy and stay at Stephany's friend Carlo's flat in summer holiday. Max is not very happy to see Carlo and Carlo's wife Ruth is not very happy to see Stephany. Stephany and Carlo say 'We are just good friends.' but...
I just taught it is hard for a man and a woman to be just good friends.