How to deal with user backlash when adding ads to your content site?

I have a private forum which has grown over the years to now have 10K registered members.

I recently added Google AdSense ads to all the forum pages. The ad appears after the first post.

How do you deal with the community disliking the ads and thinking that the site "sold out" by becoming commercial?

It's unlikely that you are getting rich off AdSense. A couple of things to consider:

1. In any community, a minority of users will always complain about any change. Expect it and don't worry too much about it
2. You may want to write a post explaining why you made that decision. Maybe the ads will help cover the cost of hosting the site. Transparency and honesty usually helps keep a community together.
3. Finally, make sure your ads are not too pushy. If you just have one ad per page, fine. Don't start putting ads all over (not to mention it's against AdSense terms of use), no sleazy tactics, etc.