Some of the common reasons were moral and religious qualms, fear of contracting a disease, lack of interest in the other person. A couple of the uncommon ones:
“Because if I want to feel badly about myself for being an overweight, out of shape single mom, I can just call my mother (and keep my clothes on too!).” (Posted by Smartypants).
“The guy kept leaping out of a coffee shop I passed on the way to work playing a guitar and singing Portugese love songs.” (Posted by Anon, who also reported saying no after hearing one of the worst come-on lines in history: “The guy told me I looked just like his exgirlfriend.”)
My favorite combo was from Michelle:
“Reason I didn’t want to: Because he was an idiot.
Reason I did: Because I am an idiot.”
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