I do a lot of flying with different people and I'm always upfront with them young & old, male and female. Always start the briefing with "ok, I get to ask you a really personal question.....how much do you weigh?" It's never been a problem as they're always told it's a safety issue. We also keep a set of scales handy if we're in any doubt (quite often being underweight in the front seat can be a problem!).
I really don't understand the attitude of pilot's who're willing to fly outside the limits of the aircraft's envelope. If you're ok to fly beyond Max AUW, are you ok to exceed VNE, G-loadings, max temps, etc, etc ? I'd be quite concerned about a pilot's judgement if he or she felt that they 'know' an aircraft better than its designer. Also be concerned if exceeding weight limits is likely to increase the fatigue rate of an airframe - so it might be fine for your flight but you've just overstressed it for the next person.
In this age of ever increasing litigation and insurers seeking any loopholes to avoid paying out, flying outside the aircraft's limits isn't worth it from a financial perspective nor from a personal responsibility viewpoint either.