Even if you're not a writer, you've most likely heard someone go off the handle and say why we shouldn't use two spaces between sentences. Here's a good example:
Slate Magazine: Space Invaders
When I was young, I learned to use two spaces. And like Farhad Manjoo says, it wasn't just me. It's what everyone learned. However, when you use proportional fonts, one space just looks nicer than two. No one can explain to me why it does, but hey ... two periods is just "totally, completely, utterly, and inarguably wrong."
Because nearly every typesetter agrees it needs to be one space -- or at least that's what they say today. But wait ... twenty years ago when it was two spaces, were all those typesetters wrong?
I don't know. I suppose I could force myself to adhere to this new arbitrary rule. The problem is that it's ingrained into my psyche, much like how I go into auto-pilot when I play the piano or drive a car. There are so many little things that we do without thinking as we concentrate on the more important things. For example, if the car in front of me comes to a sudden stop, I think, "SLAM ON THE BREAKS!" I don't think, "Let's see. I need to take my foot -- my right foot over here -- take it off of this right pedal -- and apply downward pressure on this big middle pedal -- all while pulling on the steering wheel for extra weight and screaming obscenities." Whatever happens, it happens automagically and nearly instantaneously. If I were to stop and think about the process as it happens, I'd most likely crash.
Similarly, when I type, I prefer to think about what I'm writing, and now how I'm writing it. The two spaces just happen. Does that make me an idiot? Does that make me stubborn?
I'm not sure. When I read Manjoo's article, I can't help thinking, "Wow! Here's a man who thinks he's better than 99% of the world's population -- one who wastes his time getting angry over spacing in his friends' emails."
He states, "What galls me about two-spacers isn't just their numbers. It's their certainty that they're right." But wait! I'm not the one saying, "totally, completely, utterly, and inarguably wrong." I just don't care. I'm a writer, not a typographer. You might as well be some dork trying to tell me why your religion is right and mine isn't, because ... well ... that's exactly what it feels like. All these typographers have joined up with the Church of the One Space. It took the Chicago folks till 2003 to make the change, but as of today, they've all drunk the Kool-Aid.
More than once, Manjoo complains in his article about how many spaces he's had to delete in his editing profession. Please don't tell me that he removed each one of those spaces individually -- not in these days where you can convert two spaces to one in an entire novel in less than one minute with some simple mass search-and-replace schemes. You can even set up a Word macro to do it for you. Click a button, and WHAM, no more evil double spaces.
In fact, in writing this post, I typed with double-spaces, and then quickly removed the annoying  's that Blogger so nicely puts in for you.
Going forward, I will most likely continue to type with double spaces, because:
#1) I write science fiction, and most editors take or prefer monospace font, as described by the Science Fiction & Fantasy Writers of America.
#2) As I mentioned before, it is very easy to convert to just one space. If an editor really wants me to do the conversion, it's just click-click-click, and there -- it's done. All I care is that my story gets published.
#3) It's actually more difficult to mass-edit-convert from one space to two spaces.
#4) I've seen some proportional fonts where single spaces after a period actually seems to make it harder to read. This is one of those difficult decisions I'd prefer to leave with the editor and other experts. They'll make my story look good on paper.
#5) I think I'll enjoy continuing to anger typesetters. We definitely need more people in the world that get angry over nothing. Cue the rants on facebook!
So, is it one space or two? As a writer, I just don't care.