I finished your sentence for you. That's not an anti-TKD argument. If said 15 year old girl does Muay Thai and finds herself stuck with an older ('cos let's be honest, what 15 year old girl would be seen with a boyfriend the same age - ugh!), stronger male who trains in martial arts himself, then her sole advantage (being a martial artist) disappears. For children and young women self defence is about avoiding dangerous situation and dodgy people and Muay Thai is pathetically inadequate in that field (as are most fighting arts).