Some people don't quite grasp that "I like this ship" is not the same as "I approve of this relationship in a real-world context and think it's totally healthy," and so a lot of misguided concern-trolling ensues, with said people getting all indignant at fans of sick-and-wrong ships, not realizing that those fans are probably shipping those pairings because they're sick-and-wrong. Used to happen a lot in BSG fandom with the Kara/Leoben ship, and happened quite recently in Homestuck fandom with the Gamzee/Terezi ship.