A comment in Wired UK has Jimmy Wales saying this:

Sanger was absolutely adamant that Wikipedia must have ads, and it was my refusal to do so that led to Wikipedia being as it is today. The Spanish fork did not provoke any changes of any kind. We stayed the course. I didn’t want to have advertising, and I found ways to avoid it — the Spanish fork was an important event in the history of Wikipedia, but not in the sense of “provoking change”.

The suggestion that I demanded ads and that Jimmy Wales was opposed to them is, I am afraid, yet another self-serving lie from Wales.  The lie is again, sad to say, designed to inflate his own reputation at the expense of mine.  It is also brazen, because he knows that there are at least three people–Tim Shell, Terry Foote, and me–who can provide a more accurate story.  This is not to mention words from the man himself from 2002, which you can find at the bottom of this post.
The facts of the matter are these.  Nupedia and, later, Wikipedia began life as side-projects of the ad-driven for-profit business Bomis, Inc.  It was my full-time job to start and lead both projects, while Wales was, of course, CEO of Bomis and relatively uninvolved with the projects while I was on board.  But Wales was the one to make the business decisions (along with Tim Shell, partner and co-owner of Bomis).  From the beginning, Wales let me know in no uncertain terms that, once it garnered enough traffic, Nupedia would become ad-supported.  This would of course also be true of Wikipedia when it started, because Wikipedia started as a side-project of Nupedia.  Frankly, this funding model made me nervous from the beginning, and I said so to Wales on several occasions: I was always uncomfortable with the idea of a for-profit concern, that would pay for my own job, being built on the backs of volunteers.  I still am.
At some point in 2001, it had become quite clear that the nature of the participants and the project itself was such that the encyclopedia would have to become non-profit.  This is something that I had asked for early on.
But the plan to sell ads did not change; after all, as Wales himself argued, plenty of non-profit concerns are partly supported by ads, such as PBS and National Geographic.  Our line was that the ads would be best described as low-key, unobtrusive “sponsorships” and not as “ads” per se.  The question came up from time to time about when we would start running ads (or sponsorships), but Wales’ original line, that we would not run ads until we could start making a significant amount of money from them, held good through the end of 2001.  I agreed with this, because I knew that ads would be unpopular with contributors (and made me a little uncomfortable as well; this was just before Google Ads got started).  Even when the site was approaching its first 20,000 entries and had had the benefit of coverage in The New York Times and Technology Review, traffic was still not high enough to warrant running ads.
Then, toward the end of 2001, Bomis lost a major, million-dollar contract with the old Go.com portal–all on account of the collapse of the Internet bubble–and Bomis had to lay off the half-dozen or so people they had hired in 2000 and 2001.  I was the last of these people to be laid off, Tim Shell told me.  But, around the time Shell informed me–in December 2001–that I would soon be laid off, I was also assured by Wales that he would finally have Terry Foote try to sell ads to support my position.  This was something that Wales and I discussed together and, he led me to believe, we both agreed on.  I was still uncomfortable with the idea of ads being run to support me, even in a non-profit context.  I do vaguely recall that Wales said that he didn’t think they’d have much luck, because the market for Internet ads had almost disappeared.
After a month or two, however, I had no news from Wales.  Eventually he told me that they had had no luck in selling ads, and that I shouldn’t expect money from that avenue.
I invite Jimmy Wales to offer refutations of specific claims I have made in this account.  One thing he is specifically wrong about is that he was opposed to selling ads, because doing so had been his design from the beginning of the project.  The decision not to sell ads was announced only after I had resigned from the project.  He was still far from stalwart in his opposition to ads; I went to the Wikipedia-l archives and found this mail from Jimmy Wales, which I reproduce in full:

[Wikipedia-l] Advertisements
Jimmy Wales jwales at bomis.com
Fri Mar 1 20:44:26 UTC 2002
With the resignation of Larry, there is a much less pressing need for funds.  Therefore, all plans to put advertising of any kind on the wikipedia is called off for now.
We will move forward with plans for a nonprofit foundation to own wikipedia, and possibly to solicit donations and grants to help us carry out our mission.  (Ironically, I think that grant money would come with many annoying strings attached, which we could not accept, comparted to advertising money, which is virtually 100% string-free.)
Just as the National Geographic Society is supported in large part by advertisments in the National Geographic Magazine, I expect this to be a potentially necessary thing at some point in the future, if we wish to have an impact beyond our own little corner of the Internet.  (And, I think we all do.)
But for now, there’s no pressing need unless and until we find chaos descending on us from the lack of constant oversight.
The hosting of Wikipedia I can continue to do for no charge for the foreseeable future.  Even if Wikipedia traffic were to grow by a factor of 10, I would be willing to absorb all the bandwidth and hardware costs.  If it grows beyond a factor of 100 or 1000, obviously, alternative solutions would have to be found.

So, according to Wales on March 1, 2002, just after I resigned, Wales clarifies again that there had been “plans to put advertising” on Wikipedia, but that advertisements might still become a “potentially necessary thing at some point in the future.”

And if there is any question that Jimmy Wales himself was in favor of advertisement prior to this declaration, consult this Wikipedia-L post from February 2, 2002, where he says, “However, with the ongoing hard times in the Internet economy, we do anticipate adding some forms of advertising to the site in the near future.”

UPDATE: some Wikipedians attempted to press Wales on this issue; he ended up deleting the discussion.