My great-uncle died a couple weeks ago, and his wife gave a eulogy and told this story. When they were first dating, my mom and uncle were little and my great uncle thought the world of them. Then while they were still dating, my aunt was born with a severe cleft palate and lip (my grandma had to be sedated she was so upset). My great uncle and his girlfriend went to the hospital to see them, and as he held his new niece, he saw past the deformities and kept saying how beautiful she was. And that's when his future wife knew that she wanted to marry him and have him be the father of her children.
FI says he "knew" after our first date. It took me a little longer - he got sick a few months after we were dating and I wanted so much to take care of him and make him feel better that I didn't care if I got sick too. Anyone else have stories about when they knew their FI/H was "the one?"