i want to create a search engine website using google custom search engine with own brand name. This site will include only selected domain names hosting adult p*rn content. Does google allow this?

Bear in mind that a custom search engine only displays results that are visible on Google search anyway. If your domain names are included in Google search results, I don't see that there will be a problem getting those same results via a CSE (provided of course you don't specify "safe search").