Abraham also has a child by Sarah’s slave – would you say that the Bible endorses or merely reports that event?
Back to Abraham. I would like to point out first that the teaching of Aquinas concerning the natural law as it pertains to matrimony is that he distinguishes between the first principles of the natural law which are immutable and conclusions or secondary principles as it were that follow from the first principles which though applicable in the majority of cases, in some cases may not apply and thus admit of a dispensation as it were. He gives as an example in an article treating of the natural law:
‘…Thus it is right and true for all to act according to reason: and from this principle it follows as a proper conclusion, that goods entrusted to another should be restored to their owner. Now this is true for the majority of cases: but it may happen in a particular case that it would be injurious, and therefore unreasonable, to restore goods held in trust; for instance, if they are claimed for the purpose of fighting against one’s country’ (ST, Pt. I-II, Q. 94, art.4)
This sort of thing Aquinas is talking about pertains to the positive precepts of the moral law, either the natural or the Divine Law, as these kind of precepts which inculcate acts of virtue in particular instances or under various circumstances could involve a sin. For example, if one were to stay in church all day and pray while it is his duty to support a family and work, although praying is a good act, in this case the person is failing in his duty and not following God’s will to support his family. The negative precepts of the Law, on the contrary, do not admit of exceptions, i.e, under no circumstances is murder, for example, lawful or good. These are actions which in their object are intrinsically evil and admit of no exceptions.
The conclusions drawn from the first principles of the natural law still pertain to the natural law and in those cases in which they may fail such as the example above from Aquinas, the right thing to do here in this case one could say also pertains to the natural law simply because it is the right thing to do.
With all the above in mind than, Aquinas proceeds to explain the polygamy found among the Israelites in the Old Testament. He says that monogamy and indissolubility of marriage though of the natural law perfected by Christ pertains to the conclusions drawn from the first principles of the natural law concerning marriage. Consequently, before the New Law established by Christ who is God, God could or may just simply permit or allow a dispensation in the Old Law and natural law pertaining to the conclusions or secondary principles without violating the first principles of the natural law which are immutable.