Either we have a source of authority or we do not.
For followers of Christ, that source of authority is the bible.
For those who do not know Christ, there really is no objective authority. All decisions are based on subjectivism, feelings of the moment, personal preference, and what that individual thinks "is right for me." This completely subjective morality extends to all areas of life - even marriage. When biblical restraints are rejected, anything becomes possible and even probable. Our world is now quickly embracing the gay lifestyle as not just normal but preferable. Logically, if gays can marry, then anyone can. People can even marry non-people. People can even marry avatars. Wow.