That's not a served up advertisement. It's a part of the header for the forum. It's called an announcement. By adjusting the settings in the admin control panel, you can dictate when it gets seen and by whom. If you log in you don't see it. If you're logged out everyone sees it.
The ads we're talking about are for revenue to offset the cost of running forums. I pay my fees out of pocket for mine so I don't bother with ads. It's just some times the ads here are annoying and others are funny as in who's posts they show up in.
I thought it was a little funny where this ad showed up. It's a little too risqué for younger guests and members here. So I put it here to let Janet and Andy know what sort of ads were being served up to us.