Friendship consists in a certain equality. Although therefore it is not lawful for a women to have many husbands, because this is contrary to the certainty of offspring; were it lawful for a man to have many wives: the friendship of a wife for her husband would not be freely bestowed, but servile as it were. And this argument is confirmed by experience for where the men have many wives the women are treated like slaves.
(English translation being abridged, my translation follows)
Moreover, intense friendship is not directed towards many, as is laid out in the Philosopher's Ethics VII. If therefore a wife has only one husband, but the man has many wives, there will not be a friendship of equality between both parties. Therefore it would not be a friendship of freedom, but in a certain way one of slavery.
Still more, just as it is said that marriage is to be ordered according to what coincides with good habits. It is however against good habits that one man should have many wives because discord in the domestic family follows from this situation, as experience shows. Therefore it is not fitting that one man should have many wives.
Summa Contra Gentiles III:124
For the Latinists, the original:
Amicitia in quadam aequalitate consistit. Si igitur mulieri non licet habere plures viros, quia hoc est contra certitudinem prolis; liceret autem viro habere plures uxores: non esset liberalis amicitia uxoris ad virum, sed quasi servilis. Et haec etiam ratio experimento comprobatur: quia apud viros habentes plures uxores, uxores quasi ancillariter habentur.
Praeterea. Amicitia intensa non habetur ad multos: ut patet per philosophum in VIII Ethicorum. Si igitur uxor habet unum virum tantum, vir autem habet plures uxores, non erit aequalis amicitia ex utraque parte. Non igitur erit amicitia liberalis, sed quodammodo servilis.
Amplius. Sicut dictum est, matrimonium in hominibus oportet ordinari secundum quod competit ad bonos mores. Est autem contra bonos mores quod unus habeat plures uxores: quia ex hoc sequitur discordia in domestica familia, ut experimento patet. Non est igitur conveniens quod unus homo habeat plures uxores.
Summa Contra Gentiles, III.124
Thomas' contrast between freedom and servility in marriage is an attack on extreme patriarchy, and doubtless is one among many reasons feminism could have emerged in Christian societies.
The perpetually obtuse Richard Dawkins has argued that sexual fidelity is bad because it produces jealousy. He's obviously never witnessed the machinations of a polygamous society where a few powerful men collect women. There, many men are left wifeless and despondent while the women are set at odds with one another, competing for their husband's attention for themselves and their children.
Dawkins' casual assumption that egalitarianism shall triumph in a promiscuous climate can only be attributed to naivety or blindness. A celibate 13th-century monk who once chased off a prostitute with a glowing iron poker was wiser than he.
The Ex Laodicea post in which Thomas Aquinas was cited also links to a sad essay from a struggling man who was duped by gay ideology into believing male homosexuals were actually interested in monogamy. The essay's title could describe plenty of scribbling class deviancy: The Books were a Front for the Porn