^ Pretty much this. I find that the feeling of disgust that the thought of somebody having sex with somebody else invokes in me is a good indicator of the appropriateness of the sexual interaction, although it is largely subjective and not based on logic (although I find that the two things are correlated pretty much exactly). The thought of a 40 year old fucking a baby makes me feel pretty damn disgusted. The thought of a 40 year old fucking an 11 year old makes me feel nauseous as well, although not as much so. The thought of a 40 year old fucking a willing 14 year old makes me.....not disgusted anyway.