There's actually a good number of reasons why this may be happening. One is, something, be it your computer, your internet provider, or anything in between, can be blocking the certain website due to contents that may not be suitable for everyone, asking for permission is your best solution if this is the problem. Another reason may be you anti-virus is blocking the site. You may also check if the site is hosted by a certain site (like Yahoo!) which runs an adult filter that you can simply turn off.