I had "the scare" once.
I had been dating the girl for just under a year at the time and we were pretty happy together. What had started as a relationship of convenience, we worked together and lived a block from each other, had eventually gotten serious. However, I don't think either of us really thought about spending the rest of our lives together.
I went over one day and got the patented "We need to talk" bit, so I settled down and calmly awaited being dumped. Little did I know she was about to tell me something much, much worse.
Unlike yourself, when the words "I think I may be pregnant" passed my ears I did not freak out. Instead, the exact opposite happened. My mind went entirerly blank. Honestly, that's both the first and last time in my entire life that I can recall that absolutely no thoughts were going through my head. I stared at her and she stared at me trying to guage my reaction.
Time passed and eventually she asked me what I was thinking. The sound of her voice awoke me from my "non thoughts". I got up, told her I'd be right back, and left her apartment.
I walked the block to my apartment, grabbed my pot, then walked back to her place.
I greated her upon my return to her place. I retook my seat and rolled a joint in silence. I sparked it, passed it to her, and only then started the serious discussion that went with the territory.
However, if you're looking for a reason to get out of your current realtionship, I think you found it. While terror is probobly a more common reaction than my complete and utter lack of emotion at all, the terror you felt thinking about how aweful it would be to spend the rest of your life with this particular gal should give you all the motivation you need.
Unless the sex is great. Then you're fracked.
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"The Rich are there to take all of the money & pay none of the taxes, the middle class is there to do all the work and pay all the taxes, and the poor are there to scare the crap out of the middle class." -George Carlin