How do I tell my girlfriend that I don't want to have intercourse with her because my member causes dysphoria?

I'm happy to be intimate with my girlfriend. I like her, and there's no doubt about it. But I'm a transgender woman, and I don't feel particularly comfortable when she wants me to penetrate her. I do it to please her, but it's not something that brings me truly gratifying pleasure. I only do it because that's my role as the "man" in the relationship. She expects that from me, and I assume that's how a "man" should act, even though I'm not really a "man."

Now, it's not just me doing my part in the relationship sexually. I suspect she's been looking for ways to satisfy herself with me in ways that don't require much physical demand from me (although penetration is something she seems to want to persist with). I mean, I'm not very sexually athletic, and adding the fact that I'm on spiro makes me think my sexual prowess might not be anything to write home about. To sum it up, she's understanding or tolerant of me. And I think that's precisely why I'm thinking it might be possible to change our way of having sex a little, in a way that makes me feel less dysphoric.

Any advice on how to approach this topic with her without bringing up queer or trans issues? I don't want the situation to get weird for her.