I think there's a danger with this approach. Some men dress because it excites them sexually. Some dress because they have a strong feminine identity and want to express it by dressing (among other ways). Some believe they are woman with a mismatched body and feel that is just natural to dress as a woman. I would think that your GF wants to why YOU dress rather than why others dress. "Here are lots of reasons. Pick one."
A SO might accept that her man dresses for sexual excitement, and might even use that fact to enhance bedroom activities, as long as he is still her man. She may be totally unaccepting of her crossdressing partner publicly expressing a feminine identity, wanting to increase en femme time, wanting to feminize face and body, and wanting to be her girlfriend. And, of course, the opposite reaction may happen. Why not answer the question directly?