During much of Christianity's existence, sodomy was kind of winked at. People thought they were just fine with God as long as they eventually confessed and performed the assigned penance -- until the next time around, I suppose. Yet giving up a sin is a hard thing to do. I should know.
Then again, blasphemy was supposed to be the worst of all sins, at least when it was in violation of the witness given by the Holy Ghost.
Maybe the blasphemers didn't know the Son of God, but the Sodomites tried to "know" the sons of God. In at least one of the Romance languages, the connection between knowledge and sex still exists, just like in English.
Maybe usurers want to collect interest from men, and sodomites are interested in collecting men. That probably doesn't hold up in Italian, but it would in Spanish.
Gee, I don't know. I feel like I'm circling the bait but can't quite find it.
Usury: Against Jewish and Christian law. Seen as selfish and covetous. Done even though there are people who can't afford the interest. In the story, "The Devil and Tom Walker," old Tom sold his soul to the devil. The devil commanded Tom to let his new riches out at usury. "You shall charge two percent," the devil told him. Tom replied, "Egad, I'll charge four!"
Sodomy: Against Jewish and (usually until recent decades) Christian law (or at least teachings), seen as selfish and covetous. Done even though there are women in the world who want a relationship with a man of their own.
Blasphemy: Against Jewish and Christian teachings. Seen as prideful and arrogant -- evil, even. Done even though God has told us that we can pray to learn the truth of spiritual matters. Done even though there are people who really want to know the truth, and the blasphemy can keep some of them from finding it.
I'm stumped. The only things that seem to connect them all is that a) God forbids them, and b) each of them is seen as extremely serious and bad for one's soul, if only because it harms other people at the same time it harms the offender.
Do you think that Dante was asserting an opinion that each of these sins so completely corrupts the soul that it is unlikely that the sinner will ever feel like repenting? There are a lot of bankers in the world, you know.