Observing the mitzvah of fruitfulness and multiplication

Peace and blessings
I argued with a friend about the mitzvah of pro and rabbu. The friend claimed that fulfilling the mitzvah of pro and rabbu is when you have a son and a daughter, which is a good idea in tractate Yevamot 6a. And when I asked him how there can be a commandment about the result of a mitzvah and not about doing it, he claimed that the mitzvah begins at the time of conception and is 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, the mitzvah is nullified retroactively. On the other hand, I claimed that the mitzvah of pro and rabbu is in doing the mitzvah, that is, in the time of conception and in every time of conception and in every time of conception, the mitzvah of pro is fulfilled (of course, in a time that is worthy of being sown, it is a good idea in the case of Ayalonit and the like). And the condition of a son and a daughter is that as long as you do not have a son and a daughter, you are obligated by the mitzvah, and when you have a son and a daughter, you are already free from it.
I wanted to know what the Rabbi thinks and do you have an article on the subject?