I have been dating my girlfriend for about three months. We are both in our mid 20s, grew up in the mid-western United States, and now live in New York City. She has defined boundaries for physical intimacy (i.e. waiting until marriage to have sex) in our relationship based on her religious beliefs (I was raised in the same religion, but don't follow it as closely as she does). When we start to get too close to her boundaries, she will often start to pull away from me to prevent us going beyond her boundaries.
To be clear, I would never violate her boundaries, and she knows this. We have an established pattern in our relationship of her initiating all things physical (for example she initiated our first kiss because I was hesitant to do so). We have discussed this pattern and she has told me that she doesn't think I would intentionally cross the line.
She gets embarrassed and becomes very awkward around me when she pulls away. It doesn't bother me that she pulls away, but I would like to be able to understand her so that we can proceed with our relationship in a way that she can be comfortable with. The problem is that when I try to start a conversation she becomes closed off. Typically she will turn away from me and say something along the lines of
No, it's too embarrassing.
How can I open lines of communication in a way that she will feel comfortable talking to me about such a sensitive topic?
Note: I should point out that I am on the autism spectrum (my girlfriend knows this of course). This is important information to consider when suggesting solutions.