OK I administer my network, and am familiar with the big 3 Verizon, AT&T and Sprint and don't know of any content filters. Is this a smaller provider? Do you know they are explicitly filtering content? What about your DNS server settings? (this is what translates yourwebsite.com to an IP address) any chance you are using DNS servers like OpenDNS that do content filtering? I would be pretty miffed if my ISP was filtering and that as the admin or account holder there was no access to the settings. Any chance you have addressed this with your provider and checked with them? Sometimes they will have a tool to "white list" URLs. I used to do International customer support and that was often an issue with foreign sites. Submitting URL for approval manually and then it would allow access. Your verbage is also interesting does it really say "18+" or does it more specifically indicate adult content, pornography or some such language?

NOTE: I realize in proof reading I was assuming you are in the US, please disregard information as appropriate if you are outside the US. No slight intended