Well, he passes by Lucifer’s anus, not through it. That section is about gravity reversing itself after he crosses Lucifer’s midpoint, so that he is now facing up as he climbs up and out along the devil’s legs.
Ironically, Dante was right about gravity reversing itself as one crawls through the center of a sphere, but wrong about its effects. He had gravity getting heavier as one gets closer to the center; in our world, the opposite happens.
Is the bit quoted by Mr. Kobayashi the only passus in Dante’s surviving writings - not just The Divine Comedy, mind you - that, eh, touches on the Devil’s private parts?
Is Weininger’s interpretation the only one of its kind, and unique to him, or did he base his claim on an earlier (mis)reading or (mis)interpretation?
What have others made of the possibility that Dante seems to put the Devil’s taint at the very center of the Earth, as the heart and core of all gravitation? (Or does he? Does “midpoint” mean stomach, rather than cock’n’ass?)
Dante mentions “là dove la cosciasi volge, a punto in sul grosso de l’anche.”
In Mr. Kobayashi’s quote, the line is translated as “the point at which the thigh revolves, just at the swelling of the hip.” In the page he linked to, it is translated as “the point exactly where the thigh begins, right at the haunch’s curve.” In Longfellow’s translation, it’s “where the thigh revolves, exactly on the thickness of the haunch.”
The use of the word “haunch” (“a buttock and thigh considered together, in a human or animal”) certainly seems to point in the direction of the Devil’s ass, rather than, say, his stomach. So it seems exceedingly likely that Dante is, at this point, standing on (not in) the Devil’s ass, rather than, say, hanging out in his navel.