If a man has children from a previous marriage, at least a boy and girl, is his second wife allowed, at the very beginning, to take birth control, if they both don’t want kids right away?
If not, why? What are the concerns?
There are two obligations regarding having children, the mitzvh of pru urvu, which is a biblical commandment, and then there is the mitzvah d’rabonon that even though one has already fulfilled the mitzva, never the less he should still have more children. This idea is based on the Gemora in Yevamos 62b which quotes Koheles 11-6, “In the morning sow your seed and in the evening don’t let your hand falter”, meaning that even though a person has children already when he was young he should still have more children (if possible). This is the ruling brought in the Rambam Hilchos Ishus 15-16. In fact there are opinions in the Shulchan Aruch (Even Haezer1-8) that even if a person has a boy and a girl he should still get married in order to have other children, even if it means selling a sefer torah in order to do so. This is the concern, therefore the poskim do not allow a couple to just go on to birth control without a valid reason, i.e. personal convenience etc. Another issue is that even if it is permitted, there are different types of birth control and some are forbidden because they cause waste of seed. Therefore if a couple wants to do birth control they should speak to a Rov that knows them, their situation and needs, in order to guide them correctly.