The spouse exists to serve the other's spouse's greatest good.   

"Mutuality" in any sort of contractual sense falls short of what God had in mind by conjugal union.   Marriage is much bigger than what the secular world has defined it down to be. 

The couple exists to serve something greater than itself; it exists to serve the family. 
The family exists to serve all of God's kingdom, in short God. 

Unitive serves procreative, otherwise it would be less than the perfection that God had intended. 

I realize these are hard and foreign notions to many here; but they are beautiful teachings once some generosity is given to understanding them "fully and completely".