This term usually implies an alternate state, or the nature of two things being totally contrary to one another, such as views which are stubbornly contradictory.
While the third dimension provides some interesting contrasts, we often neglect to see the commonality within what appears to be divergent, even though we use terms like "the opposite side of the street" in many ways.
Men and women are opposite sides of the same coin. The simile here for the term coin is that they are animated by spirit. They also provide for a complementary bonding in order to procreate.
The extent to which we experience and define opposites relates to our perception of physical and spiritual realities.