Well I am the opposite. I am the person who goes out a lot, who loves to spend time with people. But I think it's fine when others do not. In reading this thread I just wanted to point out the other side, that just as not every person who likes to be alone is a loner, not every person who enjoys going out and spending time with friends, is insecure and incapable of spending time by one's self.
I go to movies by myself, out for coffee by myself, but I happen to adore the company of my friends. And I have a lot of friends. Again, just because I have a lot of friends doesn't mean that those relationships are superficial. I've never had a frenemy in my life (don't even understand the concept), and while I will always work at relationships I never have to work too hard because the people I have around me are always awesome. I hate drama, I need rational friends, and I am very choosy myself, I just happen to meet a lot of people who fit what I'm looking for.
Not all writers are that stereotype of the anti-social nose in book thing, and it kind of makes you feel a bit of an outcast within your writing community when you are not. So strange really, that people who are frustrated about being misunderstood and feeling like outcasts because of their love of being alone, turn around and can be just as judgmental about those of us who are the opposite.
I think it is important to treat everyone as individuals and not generalise. If people are happy in their life, what else matters?
Also, and this isn't quite the same as what happened in your situation SP, but I was once on the recieving end of the later apology and it is a bit frustrating. You see I had gone out with my girl friend and two boys. They were both obviously hitting on her, and it was pretty funny. I wasn't interested in either of them at all (and I mean that, if you knew these guys you'd understand completely). A couple months later all four of us were remembering the outing, and one of the guys said, "I felt so bad for you that night. I mean we were both hitting on her and not paying any attention to you." And that made me a little angry. He was feeling bad for me for not having been hit on, and was giving me his pity? When I didn't care in the first place? Also they hadn't been paying attention to me? I had thought we were all getting along fine, just that my girl friend was getting more attention from them. It was just . . . well was a pretty mean thing to say and point out. Why was it necessary? It felt like a fake apology: "So sorry for leaving you out because we liked her more." Ick.
Now in your situation SP, it sounds as if the girl actually did care about the initial time you hanged out and talked books, considering her response. But there are times when one brings up an incident, and points out how worried one was that someone else was left out, when they didn't feel that way, and then they feel like, "Oh all this time they were feeling sorry for me, nice."
Anyway, might explain a bit about her defensiveness. Not that you did anything inherently wrong, but I kind of get it from the other POV.
ETA: Also don't forget, her pointing out that you like to read and she likes to go out is her trying to make herself feel better. She felt left out of a conversation because she didn't know what you were talking about, so she wanted to point out something to you that she thought you had no knowledge about. It was petty and silly, but that's probably why she did it. It had less to do with you, and more to do with her. I wouldn't let it get to you that much.