The prohibitions of immoral relations are included in Sefer Vayikra, which is also called Toras Kohanim. Although these prohibitions are not directly related to the Kohanim or the Beis Hamikdash, they deal with taharah (purity) and kedusha (sanctity). Just like the mikdash is a place for Hashem's presence, proper relationships bring His presence among the Jewish people. Every zivug (match) is divinely arranged, as each one is a unique relationship that can bring the presence of Hashem. Although we are complicated beings with physicality and spirituality joined, when we focus on another person, we can emulate Hashem and bring His presence down to our midst.