At one point, one of the old women even grabs his butt cheeks and helps him push.
Anyway, he has sex with her while these naked women chant.
So, the scene in question, is basically a mating ritual.
It’s a wild situation, and after he finishes, he escapes, fully naked, and wanders around the buildings, sometimes very aware and seemingly embarrassed by his nudity and sometimes so overwhelmed by the situation he doesn’t cover himself.