1) OVERRIDING A "SHIBUD" WITH A "NEDER"
QUESTION: The Mishnah discusses a man who makes a Neder to prohibit himself from having relations with his wife. RASHI explains that if he states in his Neder, "I am not allowed to have pleasure from you," his Neder takes effect and he is prohibited from having relations with his wife. If, however, he states in his Neder, "You may not have pleasure from me," his Neder does not take effect because he is obligated by the Torah to provide her with that benefit (the Mitzvah of "Onah") and it is not within his power to remove that obligation with a Neder.
The requirement of Onah that a man has to his wife is no more than a "Shibud," an obligation he accepted upon himself through marrying her. The Gemara earlier (59b) teaches that "Konamos (i.e. Nedarim) are like Hekdesh and are able to remove a Shibud." The Gemara there explains that although a Neder normally overrides a Shibud, the Chachamim strengthened the Shibud that a woman is obligated to fulfill for her husband, such that she cannot make a Neder to override that Shibud. Here, however, the Mishnah is not discussing the Shibud that she has towards him, but rather his Shibud towards her (which the Chachamim did not strengthen against the power of a Neder). Why, then, should his Neder not uproot that Shibud?
(a) The RASHBA here and the RAN in Nedarim (81b) explain that the Chachamim strengthened the Shibudim of both parties in the marriage. Just as a Neder cannot uproot the Shibud which a woman has towards her husband, a Neder cannot uproot the Shibud which a man has towards his wife. (See also Kesuvos 70a.)
(b) TOSFOS (59b, DH Konamos) explains that the Chachamim strengthened every Shibud and not just the mutual Shibudim between a husband and wife. Therefore, even though mid'Oraisa a man's Neder is able to override his Shibud and prohibit him from his wife, and a man's Neder is able to prohibit his money to his creditors, the Chachamim strengthened the Shibudim where one person owes something to another person and instituted that no Neder can override that Shibud. (See also Insights to 70:1:a for a discussion of Rashi's opinion in this matter.)