Why no NSFW tag? That is very clearly some folks involved in anal sex.
I’m not seeing it. All I can recognize is the vague shapes of arms and feet on the top figure. No torse or genitals. Can you explain how to see the references to sexuality in this painting?
Top figure has two pairs of legs, not arms. The pairs are oriented differently, which is why you might have seen one pair of feet and one of hands instead of two pairs of feet.
Now that you see legs, the jonction between the two pairs is more easily interpreted as sexual in nature. From top to bottom (hehe) the part with a shadow/bush, the vertical line splitting the lower pair of legs, and the small evocative horizontal line