What kind of marriage is Yukio Mishima portraying in Patriotism? What is the symbolic meaning of Shinji's and Reiko's marriage?

Shinji's and Reiko's marriage in Patriotism is rooted in tradition.

While Shinji and Reiko are relatively young, they embrace a very traditional understanding of marriage.  Their view of marriage highlights sacrifice.  While both husband and wife love one another, they see their marriage as needing to honor elements larger than themselves.  

Shinji insists that Reiko see herself as a "soldier's wife." Early on in their marriage, he tells her that she must be willing to accept the reality of sacrifice intrinsic to marrying a soldier: "A woman who had become the wife of a soldier should know and resolutely accept that her husband’s death might come at any moment. It could be tomorrow. It could be the day after."  Shinji believes that his marriage to Reiko must embrace the reality of a soldier's duty. Whatever his duty as a soldier, he must honor it.  In turn, she must honor him by living the life of a soldier's wife. 

Reiko views her marriage to Shinji as honoring this sacrifice. When he lectures her about her duties, she demonstrates that she lives to uphold such an honor. Upon revealing to Shinji the dagger her mother gave to her, it was clear that she knows what to do as a soldier's wife.  She shows that she must live her life with honor. This leaves an impact on her husband, as he "never again sought to test his wife's resolve."  Honoring her husband, Reiko lived with him as the center of her universe: "Reiko felt not the slightest surprise that a man who had been a complete stranger until a few months ago should now have become the sun about which her whole world revolved."

The marriage that Mishima depicts in Patriotism is a traditional one.  Both husband and wife live for honor.  He lives for the honor of doing his duty.  He is willing to sacrifice his life for the Imperial Forces.  She lives her life to honor her husband.  She is willing to sacrifice her life for the honor of being a solider's wife.  Their marriage is symbolic of traditional values.  It is a far cry from the modernized paradigm of marriage, where husband and wife live for their own notions of self.  In Patriotism, Mishima wants to depict a marriage that embodies traditional Japanese values.