It could be in the sense that being open to life is a matter of social justice. That of course does not mean that everyone has to have 2.1 children. There is quite a bit a diversity in the optimal number of children.
One might quibble with the OP question and point out that it doesn’t really matter how many children men have - it’s the number of children
women have, that counts (currently, globally, I understand, a fertility rate of 2.33 {children per woman} being necessary to maintain population levels)…
Anyway the 2.1 (in much of the developed world; the optimum fertility rate might be has high as 3.5 in area where child mortality, for instance, is much higher), is of course an average figure: as long as as many women have more than 2.1 (or 2.33 globally) children, as have fewer, then demographically at least the human race is set for a rosy future.
Like lots of things (climate change, helping up someone who has fallen in the road, etc), it’s not really a good idea to assume that someone else will sort everything out (ie have more children than you, to balance everything out). So in one sense maybe there is a moral duty to have children, assuming it’s a moral good to ensure that the human race survives. (I assume we are all here in agreement on this point; people who believe otherwise are rather a fringe). But otherwise I can’t construe anything that suggests anyone has a
duty to procreate, except, as others have already said, that we are all (Catholics especially, where the teaching is so explicit) called to be open to the creation of new life, and that to do otherwise ultimately is sinful.
Not sure because there is a general commandment from God to be fruitful and multiply. In any event, in our neighborhood there is a Muslim family with seven children and one more on the way. I heard that there is a similar situation in Europe and other places in the world. If Roman Catholics insist on restricting the number of children in their family to two or three, and if Muslims continue to have large families with seven or eight children per family, then what will be the result?
This “they are out-breeding us!” fear used to be said by many protestants of Roman Catholics! (and I dare say, by indigenous populations anywhere in the world colonised by Europeans).
The issue isn’t about any religion or denomination/sect, but about socio-economics: broadly speaking (and I apologise for generalising so much), many Muslim families living in the West, tend to be further down the socio-economic ladder (just as, for instance, were very many Catholic families who came to the United States 150 or 100 years ago). For reasons which are rather complex, it’s well known that poor families tend to have more children (or start having children earlier in life). While it’s little more then hearsay evidence, meanwhile. the rather middle class Muslim men or women I went to university with, generally came from families no “bigger” than my own rather WASPish one.
In other words, one should wait give things a generation or so before worrying about this sort of thing.
You can only have a duty to do something you have control over.