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To answer the question of why "thetundrawolf" thinks what you did was a bit coldhearted and shallow... All I have is your post to go on but it seems like you don't have very strong emotions about this breakup. The tone of your paragraph is just "curious" as you describe yourself. It does indeed sound a little bit robotic. It's like, get mad or something lol. The way you write it is like, "well if it works out it works out, and if it doesn't it doesn't, and I guess it didn't. that's odd. Okay, well what next?" I think that's also something that your Ex may have felt from you, some kind of extreme apathy for committing. Like if you push the SHIFT key 8 times it enables sticky keys, if she brings up marriage 8 times, Boom. Dumped.
I'm not saying she was right to force the idea of marriage so strongly. I think that where you guys are (21 and in college) is kind of ridiculous to be planning marriage. You don't know where your careers will take you yet or what your life after college will look like. So I completely get the aversion to getting engaged.
HOWEVER, just because you told her in the beginning of your relationship that you didn't want to get married doesn't mean she's not allowed to ask you about it. Also, there's a lot of grey area between "Married" and "Single." Was there no discussion of where the relationship was going to go? I can easily see her frustration here. What was your plan for the future? Do you know how many young people say they never want to get married? A lot. Do you know how many of them get married? A lot. Maybe she changed her mind over the 3-ish years you were together. That's a long time, especially at this point of your lives. You had this discussion on marriage when you were 18. From the way you portrayed it was like, she clearly wanted to get married, you didn't, she pushed it, you dumped her.