I've pretty much been preaching this for years. The only places I do freebies are Veteran's Home and hospitals and a few local benefits.
I would do one for Salvation Army if they ever asked me to.
Most of the other places that want you to work for free are for-profit businesses, such as nursing homes, retirement centers, Goodwill, etc. Many other "legitimate" charities have grossly overpaid CEO's and some channel very little of their income to the actual charity. They can afford to pay and if they want me that's what they will have to do!
As far as the YMCA, or YWCA, I know they do lots of good, but I wouldn't put up with that attitude any longer than you did!
While I'm ranting. . . you wouldn't believe how man parties or barbecues or "jams" I'm invited to attend. They they'll say, "oh and bring your musical gear so you can play for us". What if I invited a plumber or a doctor or an electrician to dinner and asked them to be sure and bring their tools?
To paraphrase Rodney Dangerfield, "We don't get no respect!". What we do is considered my most as a hobby and we ought to be glad to perform for them for free, I suppose. I'm sure they don't mean it as an insult, but they sure can be ignorant sometimes.