In Parshas Matos we read how Bnei Yisrael waged war against Midyan. Although they wiped out the men, they took the women as captives. When Moshe saw that the women had remained alive, he castigated those in charge. "Did you leave all the women alive? Behold, it was these very women who caused Bnei Yisrael, by the word of Bilam, to betray Hashem" (Devarim 31, 15-16).
Rashi explains that "the word of Bilam" refers to the scheme he proposed to Balak. Bilam told him that on the physical battlefield he stood no chance since he most certainly would not be victorious in war. Rather, he should aim to conquer Bnei Yisrael on the spiritual battlefield. Since their G-d despises promiscuity, create a situation that will cause Bnei Yisrael to transgress this sin and you will have assured yourself success. What is it about this sin that Hashem despises more than the rest of the aveiros of the Torah?
Rav Wolbe (Shiurei Chumash) quotes the Maharal who explains the severity of the three cardinal sins: murder, idol worship and adultery. The mishna in Pirkei Avos says, "The World stands upon three things: Torah, avodah, and gemilus chasadim." The three cardinal sins stand diametrically opposite the three things upon which the world stands. Therefore, one must give up his life before transgressing one of these sins. Specifically, idol worship (avodah zara) is the opposite of avodah, murder is the opposite of gamilus chasadim and adultery is the opposite of Torah.
The Maharal explains that the essence of Torah is a human being's achievement of his quintessential state. In contrast, one who commits adultery has acted like an animal. As a matter of fact, Chazal tell us that a sotah does not bring a meal offering from wheat, but rather from barley. Since she acted like an animal, her korban should be brought from a grain that is generally used as animal fodder.
This is why Hashem despises promiscuity. He singled out Bnei Yisrael as His sole nation, and gave them the Torah as a foundation and guide for how to live a life that personifies the greatness of a human being. He who behaves like an animal has eschewed the great level to which the Torah can elevate man.