http://www.domainnamesystems.com/
"Alternate Registry of the .xxx and .sex domains."
http://www2.domainnamesystems.com/xxx/info.html
since .sex does not exist and is not on the list of ssTLDs proposed to ICANN, this is highly likely to be a scam.
'alternative registry' well that's odd since ICM is the only registrar to take orders for .xxx since it wants to keep extensive records of domain owners.
My guess is 'alternative registry' means;
1. we've registered a domain and will let you buy a subdomain, On the second link it says "This page is being modified to reflect the new .xxx.us name space" so i'm guessing for your $75 you get a subdomain of .xxx.us
OR
2. Another crap program like new.net claiming to be a registry when all they do is install spyware