Depending on how marketing savvy you are, I'd go with a setting up an "Escort In Your Area" service - sort of like the Craigslist for strippers and escorts.
Now, I know you're thinking I'm crazy and it'd never work, but the site pretty much runs itself, and you just pickup the check.
Think about this:
- Take your $20k and buy some hosting on a server in Russia or the Netherlands that lets you run adult content.
- Pay a nice designer to make a very reputable look and feel (pun intended) to the website.
- Setup a tiered account system for freelancers, brothels, escorts, and pimps to pay you monthly subscriptions.
- Depending on the account type there would be additional areas - freelancers get their own page with info, escort services have sub pages for each girl, etc.
- You send out E-mails to all the reputable escort services to get them to sign up.
- Do some lead gen to attract people based on city.
- ?????
- Profit.