I don't see why they can't play as a madam. They just can't describe conducting their business on the forum. Mention she's a madam, fine. Show her setting up a transaction with a client, fine. RPing through the actual transaction, Not fine, take it to PM or fade to black. 

You can have sexual characters, you just can't publicly RP sex, any sort except in the broadest sense (forgive the following flowery prose) 

"The door closed and they moved to each other, joining as one." *fade to black* 

See?

It's a really hard thing to pin down but when in doubt you can always PM a mod and ask with examples. I promise I don't bite.