Kiddushin 51: Laws of Tithing and Kiddushin; Details on Multiple, Forbidden Kiddushin

Our last Mishna noted that when a woman and her mother or two sisters are both betrothed to a man simultaneously, neither is betrothed.  To understand this Mishna better, the Gemara considers the halachot regarding tithing.  For example, if one attempts to tithe more than the required amount, some rabbis hold that none of the tithed produce is consecrated.  This is because tithed produce cannot be mixed with ordinary produce, and as soon as tithed produce is collected it must be protected.  If it comes into contact with produce that 'should not' be tithed, it is as if all of the produce has been contaminated and cannot be sacred any longer.  

Is consummation required for a betrothal to be valid?  The rabbis continue to debate whether or not a betrothal to a woman and her sister or a woman and her mother might be a valid betrothal.  Their discussions again and again returns to the fact that it does not matter whether or not the betrothals are consummated.  They teach that some rabbis believe that all women are not betrothed (in a group of five including the forbidden pair), or that only the other three women are betrothed in this case.  This is confusing to me.  I would imagine that if the betrothal included consummation, then whomever was the first (of the mother and daughter or of the two sisters) to have intercourse with the man in question would be betrothed to him, but the second one would not.  If the act of betrothal includes the finite act of intercourse, how would a second act of intercourse, even with a forbidden person, invalidate the first act?

The rabbis even speak about an example where a man says that he will betroth the one (of the two forbidden relatives) that he is permitted to have intercourse with.  This also invalidates both betrothals, for he does not know which woman is his betrothed before he makes that statement.  The rabbis then run through a number of scenarios where a number of different daughters are being betrothed: sometimes they consider a minor girl and a grown woman and sometimes other permutations.   Further, they consider cases where a brother married to a sister dies without any children, prompting another brother(s) to step in and perform levitate marriage or chalitza.  

These cases are so far-fetched that it is difficult to remember the questions that we are considering in the first place.  One of the difficulties with halacha - or any legal thought - is that the laws must be general enough to apply to a broad range of cases but specific enough to give direction when unusual cases are introduced.  Because levitate marriage is rarely if ever practiced, and because betrothal through intercourse does not exist, and because men are no longer betrothed or married to more than one woman, today's detailed analyses seemed tiresome at best - at least, for me.