Contraception in Contemporary Orthodox Judaism

It should be pointed out that our knowledge of Rabbenu Tam’s
interpretations has always been second hand. His interpretations have
come down to us through the writings of other Rishonim – either the
various versions of the Ba’alei Tosafos or through Spanish rabbis such
the Ramban, Rashba, and Ritva. Consequently, we have different
versions of what Rabbenu Tam actually held concerning the question
at hand.
The commentary of Tosafos in both Yavamos and Kesuvos, and that of
Tosefos HaRosh in Yavamos, explained Rabbenu Tam’s position as
applying only to the moch after intercourse. Insertion of the moch
before intercourse would be forbidden by both Rabbi Meir and the
Sages, since this is not considered the normal way of intercourse and
it is similar to wasting seed (spilling it on wood and stones).
However, the compendium popularly known as Mordechai 19 has
Rabbenu Tam asking question #2 above on Rashi and rejecting
Rashi’s interpretation. He limited the debate to whether or not the
moch is recommended. No mention is made of forbidding insertion of
the moch before intercourse.
There is another source for Rabbenu Tam that may be even more
significant than the commentaries of the Baalei Tosefos. This is the Sefer
Hayashar, traditionally attributed to Rabbenu Tam himself.
Unfortunately, we no longer have the original work and must rely on
editions derived from manuscripts that were likely not written by
Rabbenu Tam 20 . In any case this is what the Sefer Hayashar (Section
165 on Niddah 45a) said about our issue (phrases in parentheses are
clarification from the authors of this article):
19 This was written by Mordechai ben Hillel Ashkenazi, a contemporary of Rabbenu Asher, the
author of the compendium known as Rosh. Both were written around the end of the 13 th century.
20 The online Jewish Encyclopedia (JewishEncyclopedia.com), under its entry of Jacob ben Meir Tam
(Rabbenu Tam) writes that the critical edition that we have available is actually the work of a student
and relative of Rabbenu Tam who was working with the now-lost manuscripts of Rabbenu Tam’s
original work. The article says that the student worked with ‘great literary precision and faithfulness’.