Why do you insist on comparing a paraphilic disorder with a normal man/woman relationship. If you can compare one pathology to sane behavior you can compare others i.e. animals, consenting children and relatives.
A married heterosexual couple? Yes. It's natural, and the way man/woman kind is built. "Enjoy the wife of your youth."
When a man and woman marry, they become one, and reproduce to form a family, and man/woman kind live on.
Group sex is out. First of all, any disease will be pased on among many. This type of behavior has proven that. Also, a womans body has an immune system designed for her husband. It just occures. Multiple sex partners is linked to uterine cance in females, prostrate cancer in men.