Kesubos 39a - Contraception

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Rashi says that the 3 women are allowed to use a moch, which applies others are not. Rabbeinu Tam asks why should it be any different that having relations with a ketana who cannot get pregnant yet it would be permitted. Rabbeinu Tam answers that the 3 women are required to use a moch, and the moch we are speaking of is one that is applied after relations. R' Betzalel Hakohen m'vilna (ma'areh kohen) points out that Rabbeinu Tam's question is answered simply by saying that these 3 women are required rather than allowed, why does he have to also say that the moch we are speaking is one of after tashmish?

He explains that Rabbeinu Tam must have understood Rashi that he also was only permiting for these 3 women the use of a moch after tashmish, so rashi would hold: 1. a moch during tashmish is assur for all. 2. a moch after tashmish is mutar only for these 3 women. Rabbeinu Tam agrees with rashi's first premise, since being meshamesh b'moch that is in place during tashmish is a problem of "matil shichvas zerah on eitzim v'avanim". The question he asks on Rashi is just as one could have relations with a ketana, so too a moch after relations should be permitted. Therefore Rabbeinu Tam says that by these 3 women wo require a moch after tashmish, but even for others is it permitted.

This is actually meduyak in Rashi 37a who explains that the moch we are speaking of is one that is used after tashmish to absorb the semen. It seems that the sugya on 37a compels rashi to say that the moch is after tashmish since it would be unlikely that it would be in place when the kidnappers would come to rape the shevuya (ya'avetz). Since Rashi interprets the use of a moch to be after tashmish on 37a, he may be assuming that also on 39a as explained above.

However, the R"I in Tosafos seems to understand Rashi that the moch which is being used is during tashmish, and is therefore forbidden for other woman (due to the eitzim v'avanim problem), but permitted for these 3 women since there is a sakana for them to get pregnant. The R"I seems to understand that although we can avoid using a moch by telling them not to have relations, they are not required to abstain, rather they can have relations and because of the pikuach nefesh concern they can use a moch.