Re: Why a man and a woman?
Marriage also demonstrates redemption, the relationship between Jesus and the Church. I would say that is its primary purpose, without the spiritual union, the physiological results are for naught.
30 For we are members of His body, of His flesh and of His bones. 31 “For this reason a man shall leave his father and mother and be joined to his wife, and the two shall become one flesh.” 32 This is a great mystery, but I speak concerning Christ and the church.
“Enter by the narrow gate; for wide is the gate and broad is the way that leads to destruction, and there are many who go in by it. 14 Because narrow is the gate and difficult is the way which leads to life, and there are few who find it." Matthew 7:13-14
(All Scripture quoted is from NKJV unless otherwise noted)