How do demons reproduce?
"In three ways they are like ministering angels: They have wings like ministering angels; and they fly from one end of the world to the other like ministering angels; and they know what will be in the future like ministering angels. And in three ways they are similar to humans: They eat and drink like humans; they multiply like humans; and they die like humans."
--Babylonian Talmud Hagigah 16a
The quote in Chagigah 16a seems self explanatory in this matter. Namely, the שדים -demons, took human form and just as humans procreate in a certain way, so to do שדים.
Alhatorah cites Pirkei D'Rabbi Eliezer (22:5) which says the following:
Rabbi Joshua said: The angels are flaming fire, as it is said, "His servants are a flaming fire" (Ps. 104:4), and fire came with the coition of flesh and blood, but did not burn the body; but when they fell from heaven, from their holy place, their strength and stature (became) like that of the sons of men, and their frame was (made of) clods of dust, as it is said, "My flesh is clothed with worms and clods of dust" (Job 7:5).
So, angels are described as flaming fire. How can fire came with the coition of flesh and blood, without burning the body? Rabbi Yehoshua in this Midrash explains that the שדים were falling angels and therefore, lost their "flaming fire" and were therefore able to procreate. Why? Because "they became like that of the sons of men".
The Daas Zkenim, in his commentary on Bamidbar 21:34 shares a interesting story:
There is also a statement by Rabi Yechiel bar Yoseph, according to whom Og had been born before the onset of the deluge, his mother having been pregnant with him at the time, and having married one of Noach’s sons, her pregnancy having been caused by one of the people whom the Torah had described as בני האלוהים, usually understood as “fallen angels.” (emphasis mine)
Maybe the fallen angels took human form and were thus able to behave exactly like a real human being.
