There is a children’s book series about similar circumstances. The Shadow Children, I think it’s called by Margaret Haddix. It’s an intriguing premise.
Assuming the conditions stated in the first post are true for the sake of discussion…
Any such law is certainly not a just law and therefore one is not morally bound to follow it out of respect for authority.
ABC is an intrinsic evil and therefore could not be used.
If they got pregnant (accidentally or intentionally) and didn’t try to hide it, I believe they would be partially culpable.
The discussion then moves to “would it be irresponsible not to abstain under such circumstances?” – a slightly different question than asking if it’s sinful.
Certainly one would be required to fight to end such a law. I believe the most responsible decision would be to abstain until the law is overturned.
If, however, there was no end to the law in sight, that puts things in a different light; abstinence during marriage is not a permanent solution. If that were the case, I would say that the moral obligation would be to flee the country.
The worst case scenario would be where one was unable to leave and had no way to fight the law. In such a case, one could either choose to abstain permanently, use NFP to avoid pregnancy accepting the chance that one could have a child, or try to raise more children secretly. The second and third options mean accepting the possibility of murder, while the first means the likely destruction of most marriages (the fundamental building block of society). I’ve never seen a more true example of being caught between a rock and a hard place.
I’m not sure what the best course of action in such a situation. My guess would be to use NFP to avoid a second child. There is a possibility of murder, but it is important to remember that the parents are not the ones commiting murder and presumably have been doing everything they could to avoid it. Under these circumstances I do not think the couple would be culpable for the murder.