I had not realized that this thread had continued with a new life!
I have absolutely no dating experience, and I realize that this has been resolved, so this is just a general question to the group, but why would someone respond to the "I had a good time" text at all if you weren't really interested? I wouldn't consider that rude, since it wasn't a question or anything that required a response, and would sort of indicate that you weren't all that eager, which might have spared the guy getting turned down for the second date.
As the OP, I felt it necessary to respond, because there is a good chance that I will see this guy again. While no one likes a rejection (of any sort), I think dropping off the face of the Earth would have created more drama than needed. He likely would have sent another message (or two) and I think it would have been worse to let him down after more time had passed. Or would have approached the mutual friend for feedback and she could have been pulled into the middle. Or I time could have passed and I could have run into him at a gathering and a "why didn't you call" scene could have played out. It could have easily turned out to be messier than it did.
But I still would have responded even if there was a much slimmer chance of seeing him again. Personally, I think being rejected sucks, but being in the unknown limbo is worse. However, I suppose we each have our preference... maybe I should open dates with "how do you prefer to be rejected?"
Someone mentioned an update... and I don't really have anything worthy of posting, but since I'm here...
He never responded to my rejection. That's fine with me. I haven't seen him since our date, but I am hopeful that things won't be too awkward if our paths do cross.
As for the mutual friend... well, I want to kick her (in that friendly sort of way.) In my OP I said that she "fixed us up", which I thought is what she was doing. (When she introduced the two of us, she dropped a HUGE hint that we were both single.) I know for a fact that my date took that to mean the same thing as I did, because he mentioned it during our evening together. Turns out that was not her intentions and she was actually thinking that he would be better suited for another friend at the party!!! Opps! Prior to accepting the date, I did check with her for approval (not that it was needed, but I thought it gave her the opportunity to give me a big warning flag if needed.) I suppose that if I was in her shoes and I had two great friends who I thought were not right for each other, I would have let things play out, too. But I wish she had somehow let me know that it was ok if we were not a match.
Anyways, when I broke the news that I had rejected his offer for a second date, she seemed to understand completely. She has never brought this up again, so hopefully all the e-Hell advice helped to make my letdown as kind and polite as possible.