I suspect that although the fake site mirrors all of your site, it has some extra stuff (maybe ads) that puts malware onto clients' computers, or maybe extra pages that are porn.
Are you sure your site hasn't had something added to it? If a common page on the site has the line:
<meta HTTP-EQUIV="REFRESH" content="0; url=http://www.anotherdomain.com/index.html">in the
<head> section near the top, then that will redirect your clients to the fake website.
To look for this, imagine you are a client and go to the normal entry page.
Then in Firefox:
Tools > Web Developer > Page source will show you the HTML source of the page.
Other browsers might differ in the details.
If OK, go to the next normal page after that, and repeat.
It sounds like there is a problem with the meet-up package that is being exploited. I recommend dropping it and using something else, and starting again with a new domain name.
Someone has reported your IP address for spam/porn/malware. You can find out more at
http://mxtoolbox.com . Your ISP should help you clear your name, or give you a new IP address.
Don't waste time trying to find out who did it - they will have covered their tracks, and in any case are probably in Bangladesh or Ukraine or somewhere.