He's already drawn Joyce porn in the past, during It's Walky. The only thing stopping him now is that it's out of character.
But even then he could just handwave it as somebody's sexual fantasy or an alternate universe, like the April Fool's drawing he did with Joyce and Glasses McSmartgirl (whose name I forgot) going down on each other.
Which reminds me, here's the April Fool's Shortpacked strip. It mostly serves to make Becky canon.
Goddammit, now I remembered that Becky exists.