( Gen 1:28 KJV): “And God blessed them, and God said unto them, Be fruitful, and multiply, and replenish the earth, and subdue it: and have dominion over the fish of the sea, and over the fowl of the air, and over every living thing that moveth upon the earth.”
God ask us to be fruitful and multiply on the surface of the earth which means that God want us to be productive and increase in number on the surface of the earth in term of plant and animal which is generally the blessing of God and have allowed the human race to be perpetuated because as one generation passes away another one appears for continuity.
but this world have now been filled with bad things, which doesn't favour the increment needed by God from man. in aspect of sex doll where people use their money to buy and want to increase in number by putting sex doll in your house. May God forgive our generation.

below i will will telling us the reasons for the topic:

WHY MUST WE BE FRUITFUL?

The following reasons are the main fact that guide Human productivity:

i. It is a command and a blessing from God for human to be fruitful and multiply on the surface of the earth. Gen 1:28
ii. It agrees with the divine principle that fruitfulness precedes multiplication. Gen 17:20; 35:11
iii For mentoring of new generations and others. II Kgs 2:9-12
iv. To enjoy a covenant blessing of overflowing fruitfulness on every side. Deut 7:12-14
v. To preach the gospel to all the world in order for people to obey God’s will. Matthew 28:18-20

GOD’S EXPECTATION IN GENUINE FRUITFULNESS:

i. Be a worthy living epistle to other brethren and believers alike. 2Corinth 3:1-3
ii. It serves as a proof and evidence for fruit of the Spirit. Gal 5:22-26
iii. Not forsaking the gathering of the brethren for solution to all problems through the world of God. Heb 10:25
iv. Covet the usage of the best spiritual gifts. 1 Corinthians 12:8-10
v. by living a holy and obedient life. Acts 8:26-31; Acts 16:25

ACCOMPANYING BLESSINGS OF FRUITFULNESS
The under-listed blessings are some of the benefits and blessings which await those who are fruitful
i. Answered to prayers. John 15:16
ii. Unspeakable joy. 1Samuel2:1; Luke 10:17
iii. fulfillment of promise. 2Tim 4:7-8
iv. power to produce more fruit which we serve God. John 15:3,5
v. Divine protection is guaranteed to ensure a continuous holy life. John 15:4-6
vi. Rewards on earth and in eternity. 1Tim 5:18, Matthew 19:27-29

It is mandatory for believers to be fruitful and multiply which is a spiritual blessing.
ii. Believers are required to meet God’s expectations in respect of genuine fruitfulness
iii. Great blessings await those who obey the commandment to be fruitful and multiply on the land
iv. it also activate a generational blessing for the fruitful ones