While on the topic of the Avos’ shmiras hamitzvos, my wife has a post on her blog on the episode of Avimelech discovering Yitzchak and Rivka “mitzachek”. Chazal derive (cited in Rashi 41:50) from our being told that Yosef’s children were born before the Egyptian famine that one is prohibited from having relations during a famine. If so, shouldn’t Yitchak and Rivka who were driven to the Plishtim’s land by famine have refrained from relations at this time?
Mizrachi answers that the prohibition applies only to areas afflicted; the Plishtim did not suffer any shortage and so no issur applied. Perhaps the issur does not apply to leil tevila (see MG”A). However, the Ta”z (O.C. 675) dismisses the whole question. The Avos were essentially Bnei Noach and were not bound to keep Torah law. What Yosef chose to do as a personal middat chassidut (Tosfos writes there is no formal issur) was not binding on others.
Based on some of our previous discussion, I don’t think it is out of bounds to suggest that the middat chassidut of refraining from relations is ascribed to Yosef as a form of asmachta to the later halachic standard, but not meant to be taken as a historical description of Yosef’s behavior. It therefore makes no sense to ask why Yitzchak did not behave in the same way. Makes sense?
One final point, which I made in passing before: do you really need to choose sides in terms of what approach you like better? I don’t see a problem is donning different hats at different moments and enjoying each approach on its own terms.