Yevamot 20: Torah Law Vs. Rabbinical Law: On Forbidden Relations and Sanctity

We are introduced to a new Mishna.  It shares a principle: When a yevama is prohibited to her yavam due to forbidden relations (Torah law), chalitza is not required and levirate marriage is forgone.  When a yevama is prohibited to her yavam because of a prohibition that is the result of a mitzva (rabbinic law) OR a prohibition that is the result of sanctity (referring to the widow to a High Priest, a divorcee, a chalutza to a priest, a mamzeret, the daughter of a mamzer, or a non-Jewish woman), she must perform chalitza to end the bond of yibum.  In these latter cases, the yevama cannot enter into levirate marriage due to the prohibition; she must perform chalitza.  Further, sisters-in-law who are also sisters must perform chalitza or enter into yibum.  

The Gemara looks at each of these categories.  Multiple cases are applied to each principle to test its worth.  Their analysis questions the application of the rival wife of an alylonit as well as that of a man who cannot have children due to damaged genitals.  The Gemara wonders about the meanings of sanctity: how might each of these women and their rival wives be affected by this Mishna?  Numerous times, the rabbis wonder how they might manage a situation where the yavam has intercourse with the yevama (they do not speak about the issue of the yevama's consent in today's daf).  Does the mitzva of intercourse (it is a mitzva, a positive commandment, according to Torah law) override the rabbinical prohibition regarding intercourse (in relationships where the rabbis determined that the union is forbidden even though it is permitted by Torah law)?

As I often wonder when learning Talmud, did people know about the intricacies of these laws?  Would a yavam who wants a 'prohibited yevama' be sophisticated enough to rape her, knowing that his action was 'sacred' according to Torah law?  I would assume that amei ha'aretz would not be aware of these differences. But the rabbis; learned men - they would know their rights.  Would these religious leaders take advantage of their power and knowledge so that they could take what they wanted?  It's legal, right?  

An interesting argument arises regarding intercourse. The first act of intercourse is a mitzva by Torah law, although it is prohibited rabbinically.  The second act, however, would be likely to follow.  That act would not be protected by Torah law if the woman is prohibited rabbinically.    Thus the rabbis declared the first act forbidden as well, as it would lead to the second act.  If the yavam had intercourse with his 'prohibited yevama', the rabbis declared that he must divorce her, as the act of intercourse is definitive as an act of acquisition.  We learn in note that Reish Lakish teaches that the positive mitzva overrides the negative mitzva if one cannot fulfill both and they contradict each other.  

It is challenging to work within legal thought that has the potential to be so flexible while it is used with such stringency.  Specifically, rape of the yevama by the yevam results in marriage.  The end.  I suppose that our legal system's tolerance of mitigating factors - intention, mental health, etc. - creates many problems for those who have been victimized, too.  But at least our system offers some acknowledgement of the emotional impact of intercourse as an act of acquisition.