Of course couples who can fulfill the mitzva should, I am just wondering to what extent it is necessary to fulfill the actual mitzva of pru u’rvu, or to help others fulfill it with public funds, when it is not in a couples natural capability. “
When you help someone fulfill a mitzvah, do you not reap the reward of knowing that your efforts resulted in the greater good? This would literally be a case of mitzvah goreres mitzvah, (loosely: one good turn deserves another). By giving tzedaka, your mitzvah could result in someone else’s mitzvha of pru urvu, and you have a chelek in it. Just as contributing to a Yeshivah gives you a chelek in the learning done by the talmidim. Only this is even more meaningful, because you are potentially helping to bring a new life into the world, which will hopefully be steeped in Torah.