I know there are tons of "best reaction" answers but I would like to focus this one on particularly mature women.
How old were you then if its okay to ask?
I lost my virginity to a friend's mom. When she took it out for the first time she just said "I knew it!" And then talked about nothing but how big I was for the rest of our encounter. There were lots of "oh my god"s and "holy shit"s. I was at an age where I had no idea that I was unusual in that department, so it was quite surprising. Especially when she kept saying how much bigger I was than her husband.
If this thread gets going there will be some fun, interesting and/or erotic mini-stories. One of those guys that got to be "one of the girls" - work in customer service many, many years now often only guy of ten to fifteen on a team. Have heard it all many times. Women over 40 or so divorced now making their way encounter an endowed guy and oh my do they have something to tell. Actually not always pro-size. Well, always pro-size if guy was a gentlemen, ha, ha.