Should my girlfriend be mad that I replied to platonic friendship over craigslist with another female?

im not trying to be mean about it, but in general i dont think anyone in a relationship should be on sites like craiglist if theyre going to be on the "casual encounters" section and things like that.i understand why she's upset, and i think you should end any kind of relationship friendship or not with the lady onnline. its also a matter if your girlfriend is more important to you than your other lady friends because honestly, if shes "the one" one random friendship with a person you barely know is not worth losing the one you love.