According to the Sefer Yetzirah (Mishna 1:5), the space continuum consists of three dimensions and the continuum of time, of two directions (past and future). As such, the Universe is constituted by four dimensions. Moreover, the Sefer Yetzirah explains that there is a fifth dimension, to wit, the moral dimension (spiritual, or soul), which possesses two directions: good and evil. This moral dimension underlines all other dimensions from the space-time continuum. The spiritual trajectory towards good or evil depends on the actions that man impresses upon the dimensions of the Universe, which reverberate in all of the olamot (spiritual worlds and this one, physical). For instance, we see this occurring in the procreative act. As it is known, “if the person sanctifies himself [and has the appropriate kavanot], he will bring a holy garment for the neshama of his child [which will be conceived through the act], and in this way, he will enable him to serve Hashem promptly. Regardless of the greatness that this soul may attain, it is nevertheless important that the father [and the mother] be sanctified during the act. But the neshama itself, apart from its ‘garments’ [i.e., the power of thought, speech, and action which clothe the neshama in its particular spiritual root and degree, which is solely decided by Hashem], is not affected by the sanctification [and kavanot] from the parents; in fact it happens at times that, the soul of a person infinitely elevated comes down and becomes the child of an ignorant and lowly person” (Tanya, Likkutei Amarim 2). However, the ‘spiritual replies’ to the actions of man on the physical level are also bound (and thus dependent) by time and its cycles, which albeit intimately connected to the physical continuum, do transcend it. This is the spiritual law of סוף מעשה במחשבה תחלה Sof ma’aseh, b’machshava tehilah “last to be created, first to be conceived”, alluding to the Bnei Israel, which establishes the precedence of time over space, regardless of its dimensional connection. Hence, certain positive actions, even when appropriate but fulfilled on ‘inappropriate times’ can be without any effect or worse yet, with the contrary effect, chaz v’shalom. For example, Kabbalah Ma’asit teaches that kameyot/kosher amulets for different purposes (e.g., protection for pregnancy, see Ta’amei Ha’Minhagim; Toldot Adam; R’ Menashe Ben Israel, Nishmat Chayim 3:25; Midrash Talpiyot 1, Efod s.v. V’HaEven; Deguel Machane Efraim, Vayicra, et. al.) need necessarily be written not only by someone with extraordinary knowledge and purity, but in precise times, that is, on specific months, days, and even hours of the day! The writing of the kameyot, which disrespects the correct times, loses its positive value, which comes from its alignment with the intermediary angelical forces that govern the five dimensions, thus rendering it incomplete (i.e., without the influence on the angelical entity) and even dangerous (i.e., by calling the attention of negative entities, G-d forbid).
And this is exactly how it was for Yosef. He knew the secrets of the illumination from kedusha/holiness called the time of “seasons, for days and years” (Bereshit 1:14) of plenitude. But he also knew well the secrets “illumination” from tumah/impurity, called the time of time of ‘seasons, for days and years’ of famine. For to know the secrets of kedusha is to know the secrets of tumah. And what is this secret of tumah? “The secret of not procreating [when the sitra achra is ‘illuminated’, to wit] on the ‘seasons, for days and years’ of famine… When the time of famine reigns, it behooves man to sexually restrict and not to allow that the sitra achra be ‘multiplied’ [through the strange souls that in this time of famine descent to the children to be] in the world, G-d forbid” (Zohar 204a, Miketz). In fact, the Ramak (R’ Moshe Cordovero) explains that, “The husband’s marital union with his wife should occur only when the Shechinah is in Its place, which means, between the ‘two arms’ [Chesed and Gevurah]. Now, when the Community is in danger [of famine] and the Shechinah is not between the two arms, these unions are prohibited” (Sefer Tomer Devorah, Perek Tet, citing the Tikkunei HaZohar from parashat Bereshit, Tikkun 69). It is for this reason that Yosef HaTzaddik made sure to procreate ‘before’ this bitter time, thus avoiding any admixture in his acts – through the use of the holy and creative power of his brit kodesh – with the sitra achra. In this manner, he truly prevented the increase of the domain of profanity that would have benefited otherwise, chaz v’shalom, from the kedusha.
(Note: it is equally not a proper time to have marital unions on a night preceding a tzom/fast save exceptions).