He specialized in mythological topics, including illustrations of Milton's Paradise Lost and Shakespeare's Midsummer Night's Dream, or Thor Battling the Midgard Serpent from Norse mythology (left).
Ok, the scene happens in the Nibelungenlied, but still, it's a rather unusual topic for a painting, unless you're a bondage enthusiast.
Maybe Fuseli just wanted to show a naked guy not having sex with a woman.
He often depicts men being accosted by predatory women. Here Satan fights with the feminine Death (odd, because Death is a masculine noun in German, der Tod).
Gay in real life, Fusili put off marriage until he was 47, when he was too old to fuss with that "sex" stuff. Later proto-feminist Mary Wollstonecraft (A Vindication of the Rights of Women) got a crush on him, and proposed a "threesome" with his wife Sofia. He refused and cut off all contact.
See also: The Gay Romantic Era