Dear Susannah Breslin:
I'm guessing that any one of the ladies at Feministing would have happily explained to you what a trigger warning actually is, since—shockingly!—it turns out that "Yahoo! Answers" isn't always the best source on the internetz. But since you didn't bother to ask them, or any of the other feminist/womanist writers in the blogosphere who use trigger warnings, let me offer my services, so that you might base your opinion of trigger warnings on Actual Facts.
It's accurate that a trigger warning is "A warning placed in the title of an e-mail or post to let possible readers know that the content might trigger (or upset) them," but that's not much use as far as explanations go when the word trigger itself hasn't been defined. (Although I note the random answerer you quoted actually did provide a bit of information in a separate response, which you chose to ignore.)
A trigger is something that evokes survived trauma or ongoing disorder. For example, a person who was raped may be "triggered," i.e. reminded of hir rape, by a graphic description of sexual assault, and that reminder may, especially if the survivor has post-traumatic stress disorder, be accompanied by anxiety, manifesting as anything ranging from mild agitation to self-mutilation to a serious panic attack.
Those of us who write about triggering topics (sexual assault, violence, detainee torture, war crimes, disordered eating, suicide, etc.) provide trigger warnings with such content because we don't want to inadvertently cause someone who's, say, sitting at her desk at work, a full-blown panic attack because she happened to read a triggering post the content of which she was unprepared for.
We provide trigger warnings because they give survivors of various stripes the option to assess whether they're in a state of mind to deal with triggering material before they stumble across it. Just like someone who isn't easily triggered can nonetheless have, say, a shorter temper when stressed or tired or hungry, a person whose history of trauma makes some material triggering for them can often navigate triggering material without a problem, except when stressed or tired or hungry. Trigger warnings give them a moment to consider whether they want to deal with potentially triggering material right now.
We provide trigger warnings because it's polite, because we don't want to be the asshole who triggered a survivor of sexual assault because of carelessness or laziness or ignorance.
We provide trigger warnings because we know that 1 out of every 6 women and 1 out of every 10 men is a survivor of sexual assault or attempted sexual assault, many of them having survived multiple sexual assaults, and just because the larger culture doesn't acknowledge the existence of this vast population of people doesn't mean we don't have to.
We provide trigger warnings because we understand what they actually are.
And now, so do you! Yay!
One hopes you will take this information on board and reconsider whether it's not that the ladies of Feministing and their readership are, in fact, too sensitive, but perhaps it's that you were simply not sensitive enough.
Because, I gotta be honest, I'm pretty sure I could make a decent case that ridiculing a feminist site for being thoughtful to survivors is evidence of not being sensitive enough with two hands tied behind my back.