Question

Hello and blessings,
I had an argument with a friend regarding the commandment of be fruitful and multiply. My friend argued that one fulfills the commandment of be fruitful and multiply when one has a son and a daughter, as stated in tractate Yevamot 61a. When I asked him how it could be that there is a command regarding the result of a commandment rather than the act itself, he claimed that the commandment begins with intercourse and remains pending until we know whether you have a son and a daughter, and if by the end of your life you do not have a son and a daughter, then the commandment is retroactively nullified. By contrast, I argued that the commandment of be fruitful and multiply lies in performing the commandment itself, namely in intercourse, and that with each and every act of intercourse one fulfills the commandment of be fruitful and multiply (of course, intercourse that is capable of leading to conception, as stated in the Talmud regarding a barren woman and the like). And the condition of a son and a daughter means that as long as you do not have a son and a daughter, you are still obligated in the commandment, and once you have a son and a daughter you are exempt from it.
I wanted to know the Rabbi’s opinion, and whether you have an article on the subject.