I have a sex issue with my MTF girlfriend

I hope it's okay to talk about sex here. Sorry if it's not.

So basically we were having sex for the first time. I'd only known for two days she was trans. At one point when we were going to start back up again from a break, she came from the bathroom and I was laying flat on my back on her bed and she got up on the bed and put her penis on my mouth.

I was 100% unprepared for that and I immediately pushed it off, then realized what just happened, and played it off ad if I was just throwing her on her back.

We finished that evening but ever since she's been slightly down. I can tell it upset her. Idk how to bring this up to her.

It really was just me being caught off guard. When I found she was trans I spent a good amount of time thinking about everything from kids to sex. I decided that I was going to love and accept her and her body. Period. I just didn't expect her to do that.

I feel like if I had a heads up that she wanted me to do do something with it, I could mentally have prepared myself to interact with another penis sexually, but I feel like I handled that moment really badly and hurt her feelings.

There's also another issue that's slightly related to this one. My girlfriend is much bigger than me down there and, honestly, it causes me to feel a little bit insecure.

Again, I hope this is okay to post here. Thanks for reading.