mitzvot lav le'henot nitnu (II)

The gemara (Nedarim 15b) discusses a case of a man who takes a neder to avoid hana’ah of relations with his wife. The Ran asks why this neder should prohibit the fulfillment of the mitzvah of onah or peru u'revu – why don’t we apply the principle of mitzvot lav le’henot nitnu (henceforth: mllh”n) and allow marital relations for the sake of the mitzvah? Ran answers that this case is very different from the case of shofar in the previous post. By the case of blowing shofar, one ostensibly receives no benefit from the act of tekiya other than fulfilling the mitzvah of shofar. However, one does have a physical benefit that comes along with the mitzvah act of onah or peru u'revu. Ran writes that mllh”n tells us that the specific benefit of being yotzei a mitzvah is not considered hana’ah, but if one receives a tangible physical benefit along with doing a mitzvah that would fall under the prohibition of the neder. Proof to this idea comes from the same sugya in Rosh haShana (28) of mllh”n – the gemara tells us that if one takes a neder from a river, one can still be toivel in the winter months for the sake of mitzvah, but during the summer where being toivel carries with it the physical hana’ah of cooling off along with the mitzvah act, being toivel would be prohibited. The Rashba answers the Ran’s question in Nedarim differently; he suggests that one can fulfill pru u’revu with another wife, and since the neder has the power to break shibudim (obligations) it also severs the marital obligation of onah between husband and wife. Apparently the Rashba did not accept the Ran’s distinction between physical hana’ah and the hana’ah of kiyum mitzvah – if not for the reasons the Rashba gave, mllh”n would indeed allow any han'ah that is associated with a mitzvah act. Aside from the proof from tevila which clearly is in the Ran’s favor, the Rashba is conceptually difficult to understand. Similar to the question we raised on the Ba’al haMaor (see previous post), we can ask of the Rashba: if mllh”n is a measure of a shiur of hana’ah, i.e. the fulfillment of the mitzvah is not a sufficient quantity of hana’ah to fall under the prohibition of the neder, how does that allow the far greater physical hana’ah which is part and parcel of a mitzvah act lie onah?