I went to Disneyland two days ago with my girlfriend and her little 12 year old brother. If I wasn't so sure I didn't want kids before, I just became a little more certain.
I don't think her brother was as bad as my girlfriend thought he was but he was acting kind of bratty. The entire time he complained about how he didn't want to wait in line (we averaged about 25-30 minutes a line which is very good at Disneyland), how he was bored, how the rides were too short, etc. He was completely unappreciative of the fact that his sister took time out of her schedule, and spent money out of her pocket, to treat her brother to a day of fun. We did our best to show him a good time but for him, that just wasn't good enough for him.
My girlfriend was quite flustered at the end of the day but I was perfectly fine with all of this until he opened my backpack to look for something and did not close the zipper. I said to him in a very nice tone, "Hey, next time you go into my backpack can you close it?" He decided to respond to me, "You close it if you want it!" Um, excuse me, no. I had been nothing but nice to this kid up to this point. We would have gone home much earlier if it were not for me saying I would take him on the rides (My girlfriend was feeling quite ill toward the middle of the day). At that point I realized that kids are just a real pain in the ass.
If there was only a 45% chance I wanted kids before, I think that just dropped 5-10%