How can I be less heteronormative?

I find myself (when talking to girlfriend) that I assume they're dating men instead of verifying first. It happened again today when talking to my roommate who is bi. I had asked about a guy she mentioned previously but she corrected me saying they were a girl not a guy * and that she's bi. Which she's told me before and I knew that but still assumed by accident. I'm trying to be better at that but still mess up sometimes. Does anyone have tips or similar experiences?
How can I be less heteronormative?
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