I think it is essentially a standard these days to spout beliefs to impress without actually taking any personal stock in the words they recite. And then there is age. 3 years is time enough to change your perspective. I certainly wasn't the same 3 years ago. 18 and 21 don't really seem like a big difference when you take a glance, but when you really think about all the things you experience in such a time frame... It's not much of a surprise for someone to be having epiphanies.
But, considering she got into another relationship within a week of a ltr, I doubt it is as complicated as that. It's both sex's fault for the state of sexuality right now. We have two modes of mind crashing into one another. We have our sexually liberated (sex is to be shared/doesn't matter), and the conservative (sex defines your purity). Each side holds onto those beliefs which define them in some significant manner, so one way or another you have to be on one side or another. Aside from us smaller minority who fall somewhere in the middle. I also feel like a lot of girls tend to follow what they deem to be a more "rare" and thus more desirable quality. But as time passes they either mature, or find a new flavor that suits them.
Femanon, by the way. This is just my own thoughts, and a bit rambly at that... but I doubt there will be many who agree.