I don't really remember sex playing any smart part in story of first two witchers except for optional cutscenes and naked cards. I don't think sex is what makes Witcher mature. It's rather overall writing. Problem is when sex and swearing or dark look slapped atop something else to make it "mature" and I think he made excellent point with Jak
It's kind of the difference that there is between, say, how sex is handled in The Witcher and Ride to Hell: Retribution.