 In the former case, there isn’t any one person who used ‘sexting’; rather, the writer is marking that it’s used in a community which they are not part of or disapprove of. In the latter case, the quotation marks are serving as emphasis. While we probably don’t want to spend a lot of time seriously theorizing about grocers’ quotes (although see Abbott 2003), whether or not scare quotes should be treated as we treat mixed quotation (indeed, whether or not they’re one and the same) is an open question (see McCullagh 2017 for discussion and an affirmative answer to these questions; Gomez-Torrente 2017 demurs).