We know in the Torah there is a commandment to have children. That is a male and female. When does one fulfill this mizvah? When the children are born or by sex? It seems to me that Tosphot holds the later opinion. The reason is this: There is n argument between th house of Hillel and the house of Shamai [that is the students of Hillel and Shamai]. Let's say you and another person own a slave. So half is owned by you and half by another. Then the another lets the slave go. [That is he write for him a שטר שחחרור]. Then at that point half the slave is a regular Jew and half is still a gentile. [I forgot to mention that a slave that is let free becomes automatically Jewish and obligated in all the mizvot of the Torah] . So at that point the slave can not have sex with a Jewess because he is still half a slave, and he can not have sex with anyone else because he is half a Jew. Thus the House of Shamai says the court of law forces you to let the slave go free so that he becomes a full Jews. The question of Tosphot is, "Why do we not say עשה דוחה לא תעשה the positive mitzvah of having children פרו ורבו pushes off the negative command of not having sex with a שפחה כנענית? " (Since the general rule is always a positive command pushes off a negative command.] Tosphot answers because the mizvah is fulfilled at the end of the sex while the negative command is violated right at הערה--the beginning.
To me it seems clear that Tosphot is thinking that be fruitful and multiply applies to every act for otherwise the question would not even begin.
To me it seems clear that Tosphot is thinking that be fruitful and multiply applies to every act for otherwise the question would not even begin.
