About the only thing I can remember laughing at in porn came from a lesbian oral vid. It was just a clip from a longer film, so I don't know the whole story, but what I could gather of the plot was that the one woman's husband had hurt her in some way, and another woman was *ahem* "comforting" her.

The scene was fine, and pretty hot, but at the end when the ladies had finished up and were kissing naked on the sofa, the husband walked in, took one look, and said, "What the fuck's going on?" A reasonable reaction, but the way he said it made it sound more like he'd walked in on his kids playing video games after bedtime, not his wife cheating on him. He just sounded way too calm, and I burst out laughing right there.