I think he's just parroting phrases used in discussions of ethics and morality and thinking it'll win him the argument. For example, in discussions of gay rights you often see "consenting adults" to describe relationships that are acceptable and exclude ones that aren't.
The difference is that's a good argument when it's about ordinary gay and lesbian couples with no power imbalance between them. It's not a good argument when it's about a punter and his victim.