The apologetics study group LOVES this quote from Groothuis’ Christian Apologetics: “We may be entitled to our own opinions, but we are not entitled to our own facts.” p. 124 It’s actually a variant of a quote commonly attributed to Daniel Patrick Moynihan and a couple others, but that is not credited in the book. Still–it is true. I think it is fair to say it has achieved meme-status.
“‘Adultery is wrong’ is true because that statement corresponds to the objective, universal and absolute moral law revealed by God, which is in accordance with his eternally stable character and the character of his creation.” p. 126
That last quote hits on natural law, divine command, Euthyphro’s Dilemma, the is-ought fallacy, et cetera. I am currently discussing that with some folks from ILP here, and have done some work on it previously if you scroll to the relevant section here (“Euthyphro, Hume, Plato, Gettier”).
Ichthus77 
How does G. prove that adultery is wrong based on objective, universal absolute moral laws revealed by God . . . in accord with the character of His creation.”
Has G. taken a good look at “the character of His creation?” There are hermit species and social species; herbivores and carnivores; some that mate for life, others that live to mate, and some that eat their mates. Plus, there are species in which sons mate with mothers, newborn children mate with each other, fathers kill other father’s children, mothers eat their children, daughters eat their mothers, and fetuses devour each other in the womb.
And for all the trumpeting of being “anti-adultery” the same OT is in favor of many thing we consider “wrong” today.
G. doesn't prove it is wrong. Justification and correspondence are two different things. Justification was beyond the scope of this chapter, and probably the scope of this book.
Morality only comes into play in species with a conscience. I agree cultures in the OT would disagree on more issues with many cultures today.
Thanks for replying, Ed.
*moral/more