The opening of the movie has questions on how they met and when they got married, but I think the ethical dilemas lie in the thick of the plot. David and Diana's gambling could be considered unethical, but it could also just be considered as stupid.
Many ethical questions lie in what each of the characters intentions are.
When Gadge offers one million dollars to spend the night with Diana, David should have questioned him. His proposal affected everyone involved, and made David uncomfortable. Everything Gadge does within the movie is questionably unethical. His intentions for the couple are questionable and he affects everyone that comes into contact with him. Diana's and David's relationship is somewhat unethical.