the reason

"The Marriage License"  - Norman Rockwell, Saturday Evening Post Cover, June 11, 1955

Not because it's historic tradition.

Not because it's aesthetically pleasing.

Not because it's personally preferred.

Not because it's socially convenient.

But because it's the design of the Creator.

But from the beginning of the creation, God ‘made them male and female.' 
‘For this reason a man shall leave his father and mother and be joined to his wife,
 and the two shall become one flesh’; so then they are no longer two, but one flesh.  
Mark 10:6-8, NKJV