^Right.
The key is their reason for not wanting children, and what God is calling them to do in that particular situation.
To use a rather extreme and equally hypothetical example: is the reason they don’t want children that they’re currently living in a war zone, are being bombed regularly, the husband has leukemia, and they haven’t eaten more than once in the last two weeks?

Well, they have several quite just reasons to avoid, but they also might not have a
duty to avoid: they should pray about it and see what God’s calling them to do.
On the other hand, and to use another extreme example, is the reason that they don’t want kids
solely the desire for each of them to purchase, with cash, a new top-of-the-line BMW each year to add to their collection? In and of itself, that’s unlikely to be a just reason to avoid.

Again, however they should pray about it and see what God’s calling them to do. Perhaps the insistence on purchasing two 50K cars each year comes from a deep-seated, unreasonable terror of financial instability, one that they need to work on in prayer and counseling? Only they and God know, which is why those with common sense mind their own business when it comes to such things.