Well, to be fair, most ladies in their 70's who go for a trial lesson ARE just wanting a one-off flight. The instructor was probably just going by experience (though it is NO excuse for being patronising!).
I told my first instructor I wanted to learn to fly but became so nervous and (mildly) airsick he didn't believe me. But I turned up for the next lesson, and the next and he treated me with every respect as a 'proper' student from then on.