Leviticus 15:24 – If a man cohabits with her, her menstruation shall be upon him, and he shall be unclean for seven days, and any bedding he lies upon, shall become unclean.
Gemora Yevamos 49b: “Her menstrual period shall be upon him”: We learn in a braita as follow – “from where do we know that if a man marries a woman while she is a niddah [during her menstrual cycle] that the marriage is still valid? From the above biblical verse ‘her menstruation period will be on him’” [implying that the marriage is valid because it will be on him]
Torah Temimah Colloquial Translation on Note #93:
This phrase of “it is/will be” ( הווייה) is according to the text of the Gemora Kidushin 68a, however in our Gemora Yevamos 49b the text used is “kidushin” [marriage]. (VERIFY THIS TRANSLATION). It appears that the textual wording of “kidushin” is the proper wording. Since the main teaching is from the phrase “her menstrual period will be on him”, even in this time of it happening, still his acquisition and her domain will be his [in other words the marriage is still good even though, actually, having relations with her at this time is subject to the very severe punishment of “excision”, or being cut off spiritually from one’s people.]
The great teaching from this is that even though having relations with a menstrual woman is punishable by excision and [the general principle is] marriage is not valid if it is with someone with whom you are liable excision (sister, etc) as is explained in the end of parsha Acharei Mot, nevertheless marrying a menstrual woman is not included in this general principle. The logic behind making an exception here is [probably] because other relations that are liable for excision are permanent prohibitions while the prohibition of having relations with a menstrual woman is temporary. Therefore, the ramifications of this prohibition are more lenient than for other relations that are also punishable by excision.
Look into what the Pnei Yehoshua writes in his halachic responsum, Chapter 3, regarding the detail of whether the marriage took place through a) a legal document b) giving money to the woman c) having relations with the woman [which is precisely what is biblically forbidden]. In my opinion, it is simple and clear that the whole point of this teaching is that even if the marriage occurred through having relations [which is forbidden] nevertheless the marriage is still valid and the prohibition is not like the prohibition with having relations with other women for whom the punishment is also excision. There would clearly be no point to the teaching if the marriage took place through the exchange of a document or money since in that case the marriage itself occurred in a permitted fashion, so of course the marriage is valid. The teaching only makes sense if it is understood to teach that even if the marriage occurred through having relations with the woman. This logic seems clear and simple to me.
However, one should note that this teaching appears not to be a clear biblical deduction from the posuk; rather the rabbis used the posuk as a “hook” for their logical deduction. This can be seen from the fact that the posuk isn’t actually about marriage at all. Rather, this teaching is one that the rabbis deduced from their logic since this woman is, in general, permitted to this man; except for the fact that there is a temporary prohibition in place [because of her menstrual status]. Therefore, the prohibition of having relations with her is not strong enough to prevent marriage. They then “hooked” this teaching onto this posuk as is their holy method.
Look also at Rambam, Chapter 18, Halacha #1, in the Laws of Prohibited Relations where he discusses the definition of a “zonah” [prostitute] who is forbidden to marry a Cohen. He answers that any woman is in a category of zonah and prohibited to the Cohen if he is prohibited from having relations with her [such as his sister, for example. Which, by the way, explains by “zonah” cannot be accurately translated as prostitute”] However, Rambam explicitly exempts a menstrual woman from this general category since the Cohen is, in fact, permitted to marry her.
According to our explanation above, Rambam’s words make perfect sense. However the הה”מ toils hard [in his commentary on Rambam] to find the source for Rambam’s ruling. According to our way of thinking, the source for Rambam’s opinion is precisely this Gemora above that marriage with a menstrual woman is halachically valid and she is not considered as being forbidden to marry. Look further above what I write concerning these rulings of Rambam at the beginning of parshat Emor on the posuk regarding a “zonah” and also in parshat Ki Tetzeh on the posuk of לא מביא אתנן זונה
Editor’s note: This note is a good illustration of rabbinic latitude and the use of a verse in the bible as a “hook” for their teaching. Also, note again that the Torah Temimah will write an explicit contrary opinion or critique of earlier teachers. In this case, the Pnei Yehoshua predated him by about 300 years.