Actually it does matter how you write it. One is definitive and the other is not. You must first define y=6 for y+3=9 or solve for said equation. The individual or individuals must understand or have prior knowledge that y=6. It must be defined. If you tell individuals y=6 then write an equation y+3=10 then y does not equal 6. How are you going to define marriage then redefine it for other applications. It is between man and woman. A union or coupling is between the same connectors/connections.