Well, I understand exactly how Buffy felt in some ways, and I hate that Parker took away her last bit of innocence. Sure, she's gone through things that have opened up her eyes to the evil in the world, and she's run away from home, and worked a crap job, and her mum has tried to burn her at the stake, and she's heard a prophecy about her death, and hey - she's died! She's gone through so much, and the last piece of innocence she had left was that she had never made a very real, very human mistake of that magnitude before.
I can understand why she was upset, and broken hearted, and why she couldn't get over it.
Also, on a completely selfish note, it's nice to see that even the most beautiful, strong, amazing girl can have this happen to her, but I suppose the question being asked here, is do I hate Parker? In short - no. With all the things that we have to hate in the Jossverse, hating Parker seems a little frivolous. The thing I hate most about him is the way he tries to pull moves on Willow, when Willow is there to tell him how much he has hurt Buffy. That seems to take it, for me, from a thoughtless one night stand, to being a thoughtless nasty person. So, a 5, I hate him a 5.