Bring it up a little at a time. My girlfriend and I were fooling around and as she was taking off my underwear, I asked her if she wanted to wear them. She shyly said “idk”. I continued “here, put them on, I think it would be sexy. And I’ll put yours on so we’ll both be doing it.” That led to a little more and a little more… now, sometimes she has me dressing up, her wearing a double sided strap-on and I ride her. It took a little while, but we got there.