No, it's because the cost it inflicts on the commons exceeds the benefit of an adolescent sex joke. If we get lots of the the latter, it will turn off and drive away some of the people we want here. Then more good users may start to leave as more good users start to leave—a vicious circle which it's basically our #1 job to (try to) make sure HN doesn't get caught in. So we're a bit hypervigilant about this.