My personal line is child rape. I write about uncomfortable things. Murder and torture are normal. I've used rape as a plot point to a number of stories, because I'm personally uncomfortable with the subject, and that discomfort comes off in my writing and makes the scene equally horrible in the exact way I want the readers to feel. But writing about the same circumstances with children is a boundary I haven't been willing to cross yet, and I don't think I ever will; it's never been necessary.
I try to steer away from anything sexual, due to this I have some difficulty with family lines at times; I intend for a character to have had sex with another character but I'm uncomfortable writing about it.
I really don't know what I would be uncomfortable writing about. I'm pretty much immune to violence and gore. For example, in my most recent story, a character got ripped apart and eaten alive by a carnivorous L-1011. I figured if I can write about that, anything is fair game.
I tend to have watchful eyes over my work, so my ability to be... off-colour in any meaning of the word is generally limited expressly. I can get away with mutilations (not of sexual organs) and some language, but nothing too much.
If I feel one of those things will go well in my story and fits in with it, I'd write it. Like Gyr said, they happen in real life whether you want it to or not. Writing about it doesn't promote it. If one doesn't feel comfortable to write about it, they have every right not to, same with if you don't feel comfortable reading about it. Yes, these are touchy topics, I understand that, so if you don't want to write this, don't. If I am faced with this, I usually just imply there was a sex scene, for example, if I don't feel alright writing it.