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I have served as a pastor-teacher and elder at a local church in the Dallas area for over 28 years. I am also a part of the bible.org staff. And, I might add, the father of five daughters -- now grown -- two of which were born while I was attending (Dallas) seminary.
It would seem that in the final analysis your actions will be judged on the basis of your motivation for not having children.
It is obvious that in your case, not having children is the result of your choice and actions -- normal marital relations would (aside from physical or medical obstacles) produce children.
I am familiar with the argument(s) that is (are) usually put forth that not haveing children is a sin for a married couple.
Are there times when preventing the birth of a child is sin? Of course. In Genesis 38 Onan sinned by preventing the birth of an heir for his brother (38:6-10). His primitive form of birth control was sin because it was his duty (which later became a part of the law) to raise up descendants for his older brother.
The question is, "Is it always our duty to marry and to produce children?"
I am not going to focus on all the arguments for marriage and child-bearing, as I'm sure you are familiar with those. Are there biblical reasons for not bearing children (even if this is the exception, rather than the rule)?
The apostle Paul seems to have chosen to set aside his liberty of having a wife and taking her along in his ministry (1 Corinthians 9:5). Earlier, Paul has clearly encouraged others to do the same, for the purpose of enhanced and unhindered service to God:
25 With regard to the question about people who have never married, I have no command from the Lord, but I give my opinion as one shown mercy by the Lord to be trustworthy. 26 Because of the impending crisis I think it best for you to remain as you are. 27 The one bound to a wife should not seek divorce. The one released from a wife should not seek marriage. 28 But if you marry, you have not sinned. And if a virgin marries, she has not sinned.
But those who marry will face difficult circumstances, and I am trying to spare you such problems. 29 And I say this, brothers and sisters: The time is short. So then those who have wives should be as those who have none, 30 those with tears like those not weeping, those who rejoice like those not rejoicing, those who buy like those without possessions, 31 those who use the world as though they were not using it to the full. For the present shape of this world is passing away. 32 And I want you to be free from concern. An unmarried man is concerned about the things of the Lord, how to please the Lord. 33 But a married man is concerned about the things of the world, how to please his wife, 34 and he is divided. An unmarried woman or a virgin is concerned about the things of the Lord, to be holy both in body and spirit. But a married woman is concerned about the things of the world, how to please her husband. 35 I am saying this for your benefit, not to place a limitation on you, but so that without distraction you may give notable and constant service to the Lord (1 Corinthians 7:25-35, NET Bible).
Here in 1 Corinthians 7 Paul does not mention child-bearing. He speaks only of marriage. Marriage has its duties and obligations (7:3-5), and these can be distractions from one's service and devotion to Christ (7:35). Thus, Paul encourages (not commands) some (not all) to consider staying single, for Christ's sake. Obviously if one chooses not to marry, then one also chooses not to bear children. I must conclude that while marriage and children are a good gift from God (Proverbs12:4; 18:22; 19:14; 31:10; 127:3), there are times when this gift can be set aside, for Christ's sake.
It seems to me that our Lord has also established this principle:
10 The disciples said to him, "If this is the case of a husband with a wife, it is better not to marry!" 11 He said to them, "Not everyone can accept this statement, except those to whom it has been given. 12 For there are some eunuchs who were that way from birth, and some who were made eunuchs by others, and some who became eunuchs for the sake of the kingdom of heaven.
The one who is able to accept this should accept it" (Matthew 19:10-12, NET Bible).
I apply this principle beyond the choice of whether or not to marry. If one, for Christ's sake, can choose not to marry, can one not also choose not to have children, or to limit the number of children they have? Surely we can agree that just as a wife can be a kind of distraction to one's service to our Lord, so children can be a distraction, and the more children, the greater the distraction.
So it is that my wife and I chose to stop having children after 5 daughters.
We could have had more, but we knew that our ministry would be restricted if we did. Some may have a dozen children, and in God's will that may be an enhancement to their ministry. But for others, children (few, or many) could be a distraction. It would be difficult for a large family to go to the jungles of South America, for example. For one thing, it costs a lot more money. For another, there are many grave dangers. A family could be a great distraction to a man seeking to proclaim the gospel in the city of Baghdad -- he not only puts his own life on the line, but the lives of his children.
So, if the principle Paul sets down in 1 Corinthians 7 (and Jesus in Matthew 19) can be applied to marriage, why can it not also be applied to the bearing of children?
Having said this, it would appear that I am agreeing with you about not having children. I'm not sure that I have heard you give this reason for preventing children. It may be your reason, but I have not heard that clearly. If we had waited to have our children until we could afford it, we would never have had any children. We had three children while I was a student in seminary. I was convicted that the husband should support his wife, and so my wife stayed at home while I worked and attended seminary as a full-time student. I say that to point out that God showed Himself powerful by providing for all our needs. I have no regrets about the choices we have made.
The story of Onan in Genesis 37 and the teachings of Jesus (Matthew 19) and Paul (1 Corinthians 7) provide us with the outer limits, I think, for our actions in the matter of marriage and child-bearing. Onan warns us about the sin of preventing children for selfish reasons; Jesus and Paul present us with the option of avoiding marriage (and, by extension, preventingchildren) for the selfless motive of serving Christ.
Make sure that what you do, you do based upon principle, and upon faith. We live in a selfish generation, in which people put off marriage and child-bearing for self-serving reasons. These are matters of personal conviction, and so others, including myself, ought not dictate your decisions in these matters (Romans 14:1-4, 22-23).