Some would claim that the propensity of men to cheat is only natural -- men have evolved to couple with as many fertile women as possible. What would a Christian theologian have to say about this? With Adam's sin, the preternatural gifts have been lost, and sense appetite is no longer subordinate to reason. This must be remedied through virtue and grace.
The male sex appetite can be stimulated by the presence of an appropriate object--that is the way appetite operates. But that does not legitimate license, or the desire or graspingness of men who want to sleep with as many women as possible. (We can describe the moral disorder in various ways: there is the role of the ego, the pride and desire to use others for self-gratification.) Just because a man may be turned on by many women does not mean that it is natural and hence moral for him to have relations with all of them.
Another example of using bad science to justify bad morals? This is the creation of a different sort of "natural" law--any desire is seen as good solely because it is a desire (and some sort of pseudo-scientific rationale can be found for it).