Read the document
Thomas Aquinas, Summa Theologiae · Supplement, q. 65, aa. 1-2
Summarized rather than quoted: plurality of wives is against the natural law only as regards marriage's secondary ends, not its primary end; and by divine dispensation the patriarchs had many wives lawfully and without sin.
Thomas Aquinas, Summa Contra Gentiles · III, ch. 124
The parallel treatment: arguments from nature for one wife, with the Old Testament plurality handled as divinely permitted.
Put it in context
Aquinas does not simply claim that Scripture forbids what earlier Scripture regulated. He reasons from natural law, the ends of marriage, and divine dispensation, and he treats the patriarchs as acting lawfully under that dispensation.
That distinction matters. His argument for one-wife practice is more developed than a missing proof text, but it also acknowledges that plurality was not intrinsically sinful in every time and circumstance.
What is disputed
The Supplement to the Summa was compiled after Aquinas's death from his earlier work, so its editorial status matters. Aquinas argues from natural law, sacrament, dispensation and ecclesial order, and reducing that to the absence of a textual prohibition understates his case.
How I read this record
This is my conclusion from the source, not part of the quotation.
Aquinas grounds the medieval rule in natural-law and dispensational reasoning rather than in a single biblical prohibition. The later popular shorthand is simpler than the argument he actually makes.
Open the sources
- Summa Theologiae, Supplement q. 65, at New Advent ↗ · Articles 1 and 2, with objections and replies.
- Summa Contra Gentiles III at New Advent ↗ · Chapter 124 in book III (via the Summa index).