This is the meaning of the verse, “And they shall become one flesh” (Genesis 2:24). The ultimate goal of Coupling is that the flesh, which is the exterior, should become one. The Ruach, the interior, actually is one – because the Ruach is rooted in unity – but the flesh does not actually become literally one. Nevertheless, under the influence of the Mental Powers, the “flesh” of the two Partzufim joins to the greatest possible degree – but only so as to be considered one flesh.
It begins with Kissing… For the Kisses come first and afterwards the Coupling of the Yesods.