Grounds For Prohibition Of Zina
I think the societies which prohibit sex outside/before marriage, have a good reason to prohibit it. Their objective is to allow the family bonding and restrict sexual acts. However, there is nothing inherently immoral in sex outside marriage as long as cheating of trust is not involved. Interestingly, Islam allows men to have sex with 4 women (in a marital context) and unlimited sex with numerous slaves outside marital context.. so if there was something inherently sacrosanct about one-man-one-woman marital sexual relations, Islam is contradicting that notion.