This may clear some things up:
1. If the mark is used "In The Same Way", it can be viewed as infringing.
2. His mark is "SexTube", not "Sex" & "Tube". Example:
http://www.googleit.com (You'd think this would be an issue, no, by the domain alone... but it is not.)
3. You cannot ? dictionary words, which "Tube" is a dictionary word considered to be fair use, and unmarkable as it is a "Generic" term.
4. HardSEXTUBE.com DOES have his mark IN IT. If it were used as Hards ExTube, it's a different story, but Hard Sex Tube would infringe. Just as you can't use DisneyTube.com or PepsiTube.com.
5. The example of Sex.com, goes back to #3. You can't have Sex?.
6. Remember BananaClaire's issue with XXXBizClaire? (It contains the mark "XBiz" in the name... as would XXXBiz)
7.
http://tess2.uspto.gov/bin/gate.exe?...002:fp6jkm.1.1 (Try to find a dictionary word in the USPTO DB that has been successfully trademarked.
8. SexTube? is a valid mark.
9. The entire purpose of a Trademark is to protect your brand from infringement from others, and to show that you have ownership of said mark. If any Joe Blow could just register any domain he/she wanted regardless of trademarks, what would be the purpose of registering them? The ICANN registry has clear rules regarding trademark violations.
10. YouTube? is the mark. You cannot go register YouTubes.com, iYouTube.com, YouTubeXXX.com etc... Just as you can't register YouSexTube.com or SexYouTube.com.
11. The TM opposition period is open for ANYONE who believes the soon-to-be-registered mark infringes on their existing mark. ie... If you own ExTube.com, and SexTube.com goes up for opposition, if you have an issue with it and think it may cause "Brand Conrfusion", then you must oppose the registration of the mark and go to mediation to explain your case.
12. The KEY POINT is that if the owner of a mark believes someone else's use of a similar mark may cause brand confusion, being in the same industry or market using similar or the same mark, it would need to be reviewed to figure out who has actual ownership and prior / first use of said mark.
Just some tips. I didn't read this entire thread, but these topics seem to be convoluted with loads of incorrect information.