I know of .com of a popular site ( that has some adult content). The .org of the site is available. Is it a good idea to book the .org of the site and monetize it ? Is the .org of a semi adult site allowed ?
No. You would be engaging in deceptive business practices. Please see a recent post for more information. Don't cloak someone else's ideas. Be original. If you want to catch their visitors, engage in a separate and unrelated business. Also, some domain hosting services ban in their Terms of Service (TOS) Agreement, adult content (i.e. GoDaddy.com). Lastly, it isn't a good idea to have adult content on a .org. It isn't banned completely, however it is a slippery slope.