Why is it forbidden to have children and marital relationships in times of calamity or difficult?
The reason is that when Klal Yisroel is going through a calamity, that we should also be in pain, (Rashi Taanis 11). On a practical level it is questionable if it applies in our present day situation, regarding the Corona Virus. the halacha is said essentially regarding a time of hunger, such we learn out from as when Yosef was in Egypt. It is questionable if this halacha applies to other difficulties, or if it is only a correct thing to do. Even if it is the correct thing to do, the poskim say that practically in our situation will largely not apply as there are a number of exemptions that would make it permitted, such as if the couple have not yet bore a son and a daughter, on the mikva night, if the wife want it. Another condition is that if abstaining will cause where the husband to have improper thoughts, then he should not abstain, and in our present day conditions this would include most people. Therefore to a large degree the poskim say that it isn’t practically applicable in our situation.
Rema O:CH 240-12, M:B ibid 46,47,48, O:CH 574-4, E:H 25-6, Yerushalmi Taanis 1-6, Yesodos Habayis U’kidushaso 8 ftnt 289. Poskim.