I used the term 'ladylike' a lot to prompt thoughts about what being a 'lady' might mean and what it might exclude.
All greetings carry expectations, whether conscious or not. Addressing a room full of people as 'ladies and gentlemen' is in part showing respect, in part requesting a certain style of behaviour from them in response. Similarly, choosing to address a room full of older teenagers as 'ladies' or 'gentlemen' conveys an acknowledgement of near-adulthood along with an expectation of adult behaviour.
You could just as well say that the OP is 'reading things into' a friendly greeting when she expresses discomfort at the man who runs her choir using 'girls' to adult women. He probably doesn't set out to patronise and belittle but he is doing so.
That's the thing with language. It's use is mostly unconscious but it has become embedded as a result of social training and experience, so it reflects societal norms and expectations. You can only start to examine and challenge those norms when you step back and consider the meaning behind what you are saying and how its use may affect others, your ideas about others and perpetuate social values. (I think that must be on day 1 of 'feminism 101').
You could say this whole thread is about people 'reading things into' perfectly nice, well-intended greetings. Or you could step back and consider why it is common to refer to adults as 'men and girls' or to women as 'ladies' and what this says about our culture and the people who use these terms.