Your girlfriend needs to read some of the wonderful hubs published by Hope Alexander on just this subject. If it is any help, I have reprinted here a declaration I wrote (mainly for myself) around 6 months ago, but have published elsewhere and also shown to some people who needed an explanation regarding my own lingerie-wearing habits.
• I am a 100% straight man, who has never entertained a homosexual thought in his life.
• I have gay and straight friends of both sexes.
• I like wearing silky lingerie, I love the way it feels against my body.
• I have no sense of gender dysphoria; I am a masculine man who loves being a man and engages in “manly” activities (watching/playing football, drinking beer, working out in the gym, admiring the female form in all its many glorious manifestations)
• I also like wearing soft, silky lingerie. Does this make me any less of a man? Does this make me a danger to society? Is this a reason for me to be ostracised by society and classified as a “weirdo”?
• Historically, many items of attire that are now considered in Western society to be “feminine”, including skirts, high heels hosiery and make-up were everyday items of male attire. Indeed, up until the middle of the 20th Century it was almost unheard of for a woman to wear bifurcated garments vis-a-vis trousers.
So, why should I have to fear the derision of narrow-minded bigots who need to overcome their own sense of inadequacy by belittling others for having proclivities and tastes that they perceive to be “abnormal”?
I hope that this has been of help to you.