TOB is about the love affair that God designed for humankind.

God desires unity with us, not just after we die.

He designed a marvelous "way" for this unity to come about.  

Actual unity is demanding.  It requires self-gift, giving up our selfishness for the great good of others. 

As JPII has said (probably a bad paraphrase, but sufficient): "by ourselves man doesn't make sense to himself!"

Our body's design points to a unity not completed.   Marriage and marital intimacy is a grand step toward the unity intended by God.

Our marriage should make it easier for God to draw us both nearer to Him!  It simplifies God's work!  

And by quick extension, having children and raising them again simplifies and progresses God's work, in a very quiet and natural way.