Seeming paradox arising from idea of eternal life

I'm thinking your definition of paradox is quite different from mine. One of the best paradoxes from the mouth of Jesus is where He said,

"Whoever seeks to save his life will lose it; whoever loses his life for my sake and for the gospel will find it."

Paradox requires, by definition, an apparent contradiction that turns out to be true and non-contradictory. Exactly what is contradictory about Christian families having three children, two children, twelve children, or no children at all?

Your use of the word paradox reminds me on people who use the term ironic indiscriminately.

"It's ironic that we had pancakes for breakfast two days in a row!"

The word they should use, more often than not, is coincidental.

That issue aside, let's talk about one drawback in having a passel of kids. First, there is no guarantee that each child is going to spend eternity in heaven with his or her parents (with all the saints, and Jesus of course). As my pastor is fond of saying,

"God does not have any grandchildren!"

In other words, each generation of children (and hence, each child) has to make up its mind to follow or not to follow Jesus. It's great when they do; it's tragic when they do not.

I'm not saying we should therefore refrain from procreating out of fear of giving birth to a soul who may spend eternity in hell. I am saying there are no guarantees that our offspring will become believers in Jesus Christ. Would to God it were so; sadly, it will not be so.

You talk about "fundamental levels," which is good. One fundamental that screams from your question is that life--every life, even the life of a baby in utero--is a sacred gift from God. While not making light of a woman who conceives as a result of rape, even the life within her has a right to be born. For her to abort that baby would not, however, be the unforgivable sin, in my opinion.

There are certainly indications in the scriptures that children are a blessing from the LORD. Moreover, the psalmist talks about the blessedness of the man whose quiver is full of arrows (i.e., children; see Psalm 127:5). Then, too, while the LORD's command to "be fruitful and multiply" is still a good command, it's just not as crucial today as it was when the entire population of the world numbered exactly two: one man and one woman!

In conclusion, I agree with you that having children is a good, albeit sobering, thing. This life we have is not all there is; there is a life--or death--to come, and for this reason each Christian couple should have only as many children as they think God wants them to have. For some, that number will be none; for others, 12; for others, only foster and/or adopted children; for still others, 2.5 (!)--I'm talking averages here ("lol," as the young people say nowadays). Your suggestion that we all have as many children as we possibly can may be well intentioned, but it misses the mark in both discernment, wisdom, and balance.

If we consider Jeremiah 1:4&5 we are told that God forms the child in the mother's womb. Therefore if God forms the child it is he who determines who will and who will not be born. Not always is pregnancy the result of intimacy, and so that alone should be enough evidence that the birth (or intended birth) of any child is a decision made by God, and not by us. We must never assume God is not in charge of all facets of life, or that we mere mortals can in any way determine anything.

Please note in 1st Samuel that Hannah was barren and that God gave a child after she implored him and also vowed to give Samuel back to the Lord.

We are entrusted with children by God, and we should as did Hannah, endeavor to give them back to God by doing our best to have them give their lives to Christ.

There are a number of theological perspectives that discuss how many children a couple should have. Those theologies are hidden under a different, much more sensational topic.

In today's modern society the question of whether to procreate or not is hidden under the question of whether contraception is moral or not. Yes, to some groups, using contraception is morally offensive, however, to the larger majority, it is more of a concern for the perpetuation of the faith. Invariably, allowing contraception leads to fewer births, which in turn leads to relatively fewer believers for the next generation. This concern is legitimate; the common phrase among those being candid about the issue is "out-breed the non-believers". The phrase is not unique to Christians either. Islamic clerics have said it on more than one occasion [Source]. It is often said and employed by racist groups as well, wanting to "out-breed" the other races¹. Even some atheists are worried they will be "out-bred" by the religious [Source].

So what does all this have to do with your question? Well, to answer according to the site rules we are here to ask and answer questions about doctrines, theologies, etc. And also to answer your question effectively for a modern audience, an examination on the various views on contraception will be very illuminating, but none of the following theologies consider the possible eternal life for the children in question. There are a few categories (According to this Wikipedia source):

1. Children in abundance - Often members of the Quiverfull Movement, those in this group believe that any contraception, even Natural Family Planning (as defined by the RCC) is morally unacceptable. They believe that the use of birth control undoes God's purpose (or one of them) for marriage.

2. Children in managed abundance - Those is this group stress the importance of having many children, however, they are more open to the outcome. Generally, you are to have however many you can conceive, but Natural Family Planning is acceptable. The Catholics are very similar to this position.

3. Children in moderation - Those in this group encourage families to have children, however, they place a higher stress on exercising prudence when family planning. As an effect, they approve of the use of artificial contraception. Generally, the number of children you have is your choice.

4. No children - Those in this group believe they may choose to live their lives without consideration or obligation to having children. There really are not many denominations that fall into this category. The nearly defunct Shakers believe procreation is immoral.

Because your question assumes that there is some sort of pressing need for Christians to procreate I will only discuss the theologies of the first three.

For the "children in abundance" group, naturally, they often quote the Genesis command to Adam and Eve, and also the repeat of that command to Noah. More commonly is that they quote

Pslams 127:3 (NASB)
Behold, children are a gift of the Lord,
     The fruit of the womb is a reward.

They argue that if children are a gift from God, then it is greatly disrespectful to refuse those gifts. You would not refuse a gift of wealth from God; likewise a gift of health and long life. So, they argue, you should not refuse a gift of children. In addition to this, they also point to a large number of verses using the phrase "open and close womb" to support that God is in control of childbearing. Those verses are: Genesis 20:18 Genesis 29:31 Genesis 30:22 Samuel 1:5-6 and Isaiah 66:9. To answer "Why does God command ..." is clear. It is because He wants to bless us and give us the precious gifts of children.

Now, it seems to me, that the "children in managed abundance" and "children in moderation" groups are very similar to the first group. They will certainly agree that children are a gift and that God does, indeed, control the miracle of conception, but they consider at least this one thing: care for the children. The use of the word "prudence" by some in the third group implies that the welfare of the child should be considered when family planning, and there are circumstances when a new child under your care would put unnecessary hardship on the parents, the child, or both. The difference between groups two and three just seems to be the degree to which "prudence" is applied. However, I would note that the cultural "out-breed the non-believer" stance may be a factor in group two's beliefs, but likely not in group three's.

How would groups two and three answer "Why does God command ...?" Group two may reference the blessing verse, but group three certainly would not. In fact, what is more likely is that both groups would not necessarily agree that it is a command for all mankind. Some would even likely quote Paul saying "It is good for a man not to marry," (See 1 Corinthians 7 (NIV)) pointing out that neither him, nor Jesus married nor sired any children.

Wikipedia (again) has a section discussing denominational positions on contraception.

1. An anecdote. My wife worked with a man that admitted freely he believed in white racial supremacy and his church (likely very small) taught that they must "out-breed" the three M's: Mormons, Muslims, and Mexicans. Clearly, the first two could be taken without racism implied, however, not so much for the third.

This answer was adapted from a previous answer I wrote on a similar question.