Because not everyone is able to meet their partner.
With God, you can find the one you belong with . . . not necessarily the one you would choose, but with God it will turn out so much more and better of more, than whatever you the way you are mature now could want and look for.
After you mature more with one another, then can be when you will do some real discovering of how love is done.
So, trust God about it
God is more "mature" and good and smart than we are, and He started marriage; and nowhere does He say to stop being married because we have had children and turned them loose, or have never had children. Either way, they need our example.
Like I understand you are saying, reproduction is not only physical; be we need to bring up children who know how to love. And in case we do not have our own, we can minister to others who do and their children.
"casting all your care upon Him, for He cares for you." (1 Peter 5:7)
So, we should not limit God to what we are capable and mature enough to know and understand now.
Sure you can argue the genesis mandate but that's old testament, they're enough unbelievers for you to convert, you need not your own bloodline to be Christian.
This is true, in case I understand you correctly. it seems you are saying you can reproduce children of God, by reaching and ministering salvation to lost people - - - so they become our Father's adopted children in Jesus.
However . . . God is Father and Son and Holy Spirit > family. And He does still have families . . . where we can learn how to love and share as his family, and minister this to other families and unmarried people. Families, then, can be used by God to help us all learn how to love and share as family in Jesus. This could be part of why our Aposte Paul's qualifications for a "bishop" include how a "bishop" qualified is a family man > 1 Timothy 3:1-10.
Also there's also practical issue like can't meet with someone you're into. By that it's ok to stay single. It makes no sense for the sake of being married you marry someone you're not interested in.
Guided by God > this works better than I look for someone who meets my requirements which I am now capable of having as a less mature person. We need to become able to love as God's family with various people of many interests . . . and not get bored and lonely in the process. Because God who is all-loving has adopted all of us.
And our Apostle Paul became as ones he was reaching to > 1 Corinthians 9:19-23. So, from this I see Paul did not pick and choose who had his interests, but he could adapt to each person, in order to communicate with the person. He would take an interest in anything that mattered to someone so he could communicate, and then bring the person to Jesus and Jesus Christ's interests
