[Trans Sex] My girlfriend is a trans woman. How do I ask her to dominate me in bed?

So, my girlfriend of three months came out to me as a transgender a week ago, and how she was not getting surgery to change her genitals. At first, I was very put off by the idea of her having a penis. Last night, I had a dream that I was sucking her off, and she put her penis in my ass. I woke up and was so turned on by the idea. I want her to dominate me, but I'm embarrassed to approach her on the topic in fear that she will be offended that I'm asking her to be the dominant one during sex. How should I go about asking her?