I've already said that in later Muslim jurisdictions Harems and concubines were indeed enjoyed by 'Muslim' rulers. Notably the Mughals - who also justified drinking alcohol and other hedonistic practices. Some of the rulers were more religious than others, but as with all scripture - we have examples of those who misinterpret. Take your Christian terrorist group - the LRA - they kidnap and rape young girls, and enslave them. You condone this action when Israelites do it - as described in the Bible, and you consider the LRA to be Christian.
However, the point is that God does not say that adultery is not a sin. Kung does not say that Islam allows adultery either.
On the contrary on pg155 Kung states:
By contrast, the Quran decisively affirms the institutions of marriage and family.
and in pg 156
Se.xual intercourse outside of marriage is not allowed.
And then he goes on to explain the concept of polygamy in Islam.
But I'm glad you're reading the book - perhaps you'll be able to quote Kung's conclusions about whether the God fo Islam is any different from the God Jesus prayed to.
Cheers,
Shafique