NICE COMMENT ON A BLOG "All that really had to be said was that almost everyone at some point in their life is tempted to sexual immorality. It is a common temptation and a common sin. In fact, it fills the world with sinners. If it fills the world with sinners, then it must also fill Hell with the condemned. That form of sin is tremendously displeasing to God who, for the reason of impurity only and none other, destroyed cities in Old Testament times. Now that is some talk worthy of any traditional outlook."
I would agree but also add that idolatry is also on the top of the list of things that God is unhappy with. And the religious world is full of it.
In any case just for information's sake, generally in the Old Testament: the forbidden relations in Leviticus are on a whole different plane than other kinds of relations. For example the prohibition of a mamzer bastard marrying into the congregation of God is a prohibition, not a כרת [being "cut off from one's people"]
Moav and Ammon are along the same lines. Just prohibitions. Sex with an unmarried girl is required to marry the girl unless she or her father object. If they object, then he pays the normal amount of a Ketubah 200 zuz.
Most of what people consider sinful in terms of sex is very different from the view of God. For example the menstruating woman problem is right in there in Leviticus 18 and 20 among the עריות. Homosexuality is also right there among the prohibitions one has to give up his life rather than transgress.
People also get confused about the betrothed unmarried girl in Deuteronomy and why is she any different than the unmarried girl in Exodus? The reason is this betrothal is not what people think nowadays. It is marriage. That is marriage always has two parts. קידושין ונישואין. Kidushin and Nisuin (or what is called Hupa). The Kidushin [betrothal] makes her already married, but they do not live together until Nisuin.
I hope this makes things clear. I ought to add that Rav Nahman brings problem of spilling seed in vain from mystic books and brings his own correction for that [to dip in the ocean -total immersion.] and ten psalms. But that is not a prohibition anywhere near the scale of things actually forbidden in the Torah. The sexual thing is mainly about things the Torah considers very evil are thought nowadays to be fine. And things that are actually perfectly OK in Torah are thought to be terrible sins.
[The so called sex change is not a change in sex. It is extending skin to make it seem something it is not or the opposite.]