I know the Qur'an says no adultery is allowed, and it's classed as the 3rd worst sin someone can commit in Islam (after 1.shirk, 2.murder), but I don't understand the reasoning behind it.
The only counter argument I can think of is that it would cause the spread of STIs or babies born out of wedlock. However, using a condom is the answer to that and not having too many sexual partners keeps one safe.
Why is it such a serious crime in Islam??!! It's not harming anyone...
It is also about self control. If we do not not restrain ourselves this world will deteriorate into a living hell. It is so easy to let yourself go and commit zina and adultery, however it is so hard to control this urge and those who do control it are rewarded. Unlawful sexual urges are from shaitan beginning with unclean thoughts and desires and culminating in fornication and adultery. A virgin who can stay chaste until marriage is also pure and purity is only from Allah Subhana Wa Ta'ala.