Your suggestions fail to make sense for the following reasons:
1) xxx is language-independent unlike your suggestions which are in English.
2) Something might be of sexual nature and deemed unsuitable for kids without being pornographic, hence ".porn" does not make sense.
3) Something might be targeted to adults, or else censored for adults (eg involving violence) and not be of sexual nature, hence ".adult" would be far too broad a term for what they are trying to achieve.
4) Something might be of sexual nature without involving sex, hence ".sex" might be confusing.
5) Regarding different cultures, I strongly believe that the meaning of a word lies strictly in its use. Once the Internet starts using .xxx, cultures will adapt and it will be universally understood as so.