What is the problem of having marital union on nights other than Shabbat (and special dates)?

It is important to understand that the halachot in general, and the ones guiding the couple’s sexuality in particular, are even more stringent and deep when treated with the benefit of the hiddurim (the various embellishments) of Kabbalah. The ‘ideal’ moment for the couple with a true Jewish soul to be united is on the night of Shabbat, for on this time, “the neshamot are distributed for the initiated on the mystical Wisdom [i.e., the B’nei Torah]… On this night, a holy and sublime ruach descends to those who are ‘sanctified’ [in order to fulfill the mitzvah of procreation, saying the prayers before the act, ‘both’ ritually washing their hands and giving tzedaka]… Thus, for these men of wisdom, the appropriate time for this ‘holy function’ is when this exalted ruach is diffused amongst them… And it is in this manner [and in this time] that the tzaddikim ‘transmit’ their holy souls for the new conception” (Zohar II:204b, Vayak’hel). Al pi halacha, “all nights are appropriate, in particular if the act occurs at midnight [when the Hole One, blessed be He, rejoices with the tzaddikim in the Gan Eden]… However, [it is important to know that] on the other nights, ‘another’ type of ruach hovers” (ibid. Zohar). This means that, “the joy of fulfilling this mitzvah is complete on the nights of Shabbat, when the person does not need to worry if there are any admixtures from the sitra achra in the conception” (ibid. Zohar). On the other nights (save on those prescribed by the halacha) it is possible that instead of simple joy for the mitzvah, there is also “the deliberate desire and awakening for the simple pleasure to give in to lust, to wit, without the real ‘need‘ to fulfill this holy act. This means that he is only following the advice and force of his yetzer hara” (Kitzur Shulchan Aruch, Hilchot Tsiniut), which is vitalized, after all, by the sitra achra.