Objection 1. It would seem that Adam's sin was more grievous than Eve's. For it is written (1 Timothy 2:14): "Adam was not seduced, but the woman being seduced was in the transgression": and so it would seem that the woman sinned through ignorance, but the man through assured knowledge. Now the latter is the graver sin, according to Luke 12:47-48, "That servant who knew the will of his lord... and did not according to his will, shall be beaten with many stripes: but he that knew not, and did things worthy of stripes, shall be beaten with few stripes." Therefore Adam's sin was more grievous than Eve's.
Objection 2. Further, Augustine says (De Decem Chordis 3 [Serm. ix; xcvi de Temp.]): "If the man is the head, he should live better, and give an example of good deeds to his wife, that she may imitate him." Now he who ought to do better, sins more grievously, if he commit a sin. Therefore Adam sinned more grievously than Eve.
Objection 3. Further, the sin against the Holy Ghost would seem to be the most grievous. Now Adam, apparently, sinned against the Holy Ghost, because while sinning he relied on God's mercy [Cf. 21, 2, Objection 3]. St. Thomas is evidently alluding to the words of Peter Lombard quoted there], and this pertains to the sin of presumption. Therefore it seems that Adam sinned more grievously than Eve.
On the contrary, Punishment corresponds to guilt. Now the woman was more grievously punished than the man, as appears from Genesis 3. Therefore she sinned more grievously than the man.
I answer that, As stated (3), the gravity of a sin depends on the species rather than on a circumstance of that sin. Accordingly we must assert that, if we consider the condition attaching to these persons, the man's sin is the more grievous, because he was more perfect than the woman.
As regards the genus itself of the sin, the sin of each is considered to be equal, for each sinned by pride. Hence Augustine says (Gen. ad lit. xi, 35): "Eve in excusing herself betrays disparity of sex, though parity of pride."
But as regards the species of pride, the woman sinned more grievously, for three reasons. First, because she was more puffed up than the man. For the woman believed in the serpent's persuasive words, namely that God had forbidden them to eat of the tree, lest they should become like to Him; so that in wishing to attain to God's likeness by eating of the forbidden fruit, her pride rose to the height of desiring to obtain something against God's will. On the other hand, the man did not believe this to be true; wherefore he did not wish to attain to God's likeness against God's will: but his pride consisted in wishing to attain thereto by his own power. Secondly, the woman not only herself sinned, but suggested sin to the man; wherefore she sinned against both God and her neighbor. Thirdly, the man's sin was diminished by the fact that, as Augustine says (Gen. ad lit. xi, 42), "he consented to the sin out of a certain friendly good-will, on account of which a man sometimes will offend God rather than make an enemy of his friend. That he ought not to have done so is shown by the just issue of the Divine sentence."
It is therefore evident that the woman's sin was more grievous than the man's.
Reply to Objection 1. The woman was deceived because she was first of all puffed up with pride. Wherefore her ignorance did not excuse, but aggravated her sin, in so far as it was the cause of her being puffed up with still greater pride.
Reply to Objection 2. This argument considers the circumstance of personal condition, on account of which the man's sin was more grievous than the woman's.
Reply to Objection 3. The man's reliance on God's mercy did not reach to contempt of God's justice, wherein consists the sin against the Holy Ghost, but as Augustine says (Gen. ad lit. xi [De Civ. Dei xiv, 11), it was due to the fact that, "having had no experience of God's severity, he thought the sin to be venial," i.e. easily forgiven [Cf. I-II, 89, 3, ad 1].
Source: Summa Theologica (New Advent)