If nothing else, it highlights, an (at least) unconscious bias on your part. Language matters, and you can work on it.

Boyfriend/girlfriend labels are equitable, and that relationship implies a familiarity for which a diminutive is typical and appropriate. Notably, this is not a professional relationship.

Girls in tech is about recruiting and supporting female children interested in tech. That's okay, because they're children. Your coworker is an adult, and it behooves you to treat them like one. For example, if she isn't acting like an adult, that's an issue to address in a professional manner, rather than call her a girl behind her back.