I think the implication is the woman next to him watching the video is his wife and mother of his child. And she just learned he has a "girlfriend".
Alternatively, the video might be NTR fetish porn and not real. In which case the wife's expression indicates disgust rather than betrayal.
Yeah this one was bothering me too. I figured it was something like that because the twists this artist does are usually funny in some way. The punchline being she's grossed out by him being into this porn is better than just straight up cheating.
