FWIW, a few thoughts of my own here...
1.) In an online game, it is very difficult to accurately "gauge" your players. Hence, I think the suggestion to exclude mature content from a game unless it is explicitly stated upfront that such will exist is a good one.
2.) There's a difference between "maturity" and "experience." A 19-year old can be "more mature" than a 30-year old. However (assuming normal lives), a 19-year-old is never "more experienced" than a 30-year-old (in one or two specific things, perhaps, but overall depth of experience is limited). You can only gain experience with time - there are no shortcuts. I was mature at about 8 (I was often called 8 going on 18). I could handle subjects like violence, rape, sex, abuse, kidnapping, murder, and so forth and discuss them in a rational, intellectual manner with adults of any age. But I sure wasn't experienced! Experience naturally alters your perspective.
3.) Do I think Xarlen made a mistake, or that the player made a mistake? Probably not. If this was the first time "mature' subject matter has been introduced, and if it was introduced without warning, then that was a mistake on Xarlen's part. Otherwise, the handling of the situation on both sides looks pretty good... you have a DM who genuinely feels bad that he may have inadvertently stepped on the feelings of another and you have a player who didn't storm off, but sucked it up, played through it, and then let the DM know after the fact that he had crossed the line.
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As others have said, it's almost impossible to know exactly what will touch nerves with people. Usually those who have been through traumatic experiences have no desire to revisit them again. If I knew one of my players suffered sexual abuse as a child, for example, I would not throw "sexual abuse" (of children or otherwise) into a mature game (not that I would run a mature game in the first place, but for sake of argument) without first having a long talk with the player, exploring the depth of the scars and asking if it is okay to deal with such issues. Ditto for "miscarriage/stillborn/loss of child" or "physical abuse by drunken father" (which my father had to deal with; his earliest memory as a child is that of hiding under his bed while his father stalked through the house with a knife, looking for a "disobedient brat" to cut up) nor even "childhood sickness ravages body" (my mother-in-law had polio and is confined to a wheelchair). I might even shy away from "divorce"-centered stuff.
Why? Because the game is supposed to be fun - people play the game to escape from the "Bad Stuff" in their real lives for a while and take up other, more manageable problems; though occasional exploration of a complex issue is desirable, it's probably good to make sure that's not the norm. In fact, by default, I don't bring any of the above issues up unless I know every single one of my players well enough to know for darn sure it's not going to touch a nerve.
I am reminded of an incident when *I* was 19 - I had a female friend call me up near midnight and tell me she had just been in a car accident and miscarried a baby (she hadn't told her parents she was pregnant in the first place; that's why she called me - but living on the opposite end of the country with the navy for a year and having just transferred to a new base, she didn't know anyone there she felt she could call - and I gave her my number before she left and told her if she ever needed to talk about anything, I was "there" - and I suppose she felt she needed to use that). I told her I was very sorry to hear that and that I felt awful for her. I said something along the lines of, "calm down, relax, you're all right... I understand..."
She cut me off, "YOU don't understand ANYTHING! You haven't had a child growing inside you! You haven't felt it kicking! You have NO IDEA what I'm going through right now. You're a man! You can NEVER understand this!"
And you know what? She was right. I didn't. I couldn't. I can't. I told her, "You're right. I don't understand exactly what you're going through. I DO understand that you are going through something that's really difficult and are looking for comfort, otherwise you wouldn't have called me. While I don't understand how it feels to go through it, I can understand that it feels horrible."
With my wife and myself expecting our second child at the end of the month, I think I have a little better understanding. When my wife was about 7.5 months pregnant with our first child, she was still working. One Saturday, in the middle of my weekly D&D game, she called me.
"Honey, we need to go to the doctor now. I haven't felt the baby move at all for two days now."
I went pale. I hung up the phone. We were literally in the middle of a combat round. I said, "guys, game's over right now. I have to go.") They knew something serious was up. On the drive to pick up my wife, I had plenty of time to review worst-case scenarios in my head. That was probably the closest I could come to understanding a miscarriage... the baby, of course, started kicking up a storm pretty much as soon as she hung up the phone with me (little stinker :b - but of course, I can laugh about it because everything was all right in the end).
Now, my son has had a couple of "close calls" since - he was down to 4.5 pounds when we took him home from the hospital after being born two weeks LATE and wasn't eating well. Lots of sleepless nights there. At about a month old, he got sick and went back to the hospital for almost a week. Those "close calls" have given me some idea of what it might be like to lose a child - but I still don't truly *KNOW*.
You might be in the same situation with your player. She may have that same thought - "YOU don't understand ANYTHING!" And she may have a point.
Ultimately, I agree with the advice given - catch her privately online and explain to her you are sorry if you hit a little too close to home on that one - while you don't understand the depth of sorrow it might cause, you do understand that it hurts a lot and didn't know anyone might have a problem with it - and you feel bad that it did cause a problem and possibly reopen old wounds. You won't do it again. In my experience, most people are quite willing to forgive you if you naively touched a nerve. Obviously, if you keep bringing it up, you should expect less sympathy. But I think everyone's entitled to "miss" once on stuff... after all, you don't know the first time you try it.
There. My 2 cents.
--The Sigil