It depends.
If the name of your site is xxx-porn-site.whatever then you can't trademark it because those are generic terms. You can't trademark coffee or blue jeans either. If you're name is Bob Aardvark Smith and you run bob-aardvark-smiths-homemade-xxx.whatever then you can trademark the name and stop people from starting any adult site with Bob Aardvark Smith in the title.
The real question is do you have the deep pockets not only to obtain the trademark but defend it?
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