Why am I so weirded out knowing about my childhood friends sex life?

I [18M] recently ran into one my childhood [18F] friends again in college for the first time in eight years. I didn’t really use social media at all during that time so I hardly talked to her. We’re now friends again and I’ve started hanging out with her friend group. I swear I have ZERO interest in her romantically or sexually, I’d just like to be friends like we used to be.

Basically, I was sitting a few rows in front of her in a lecture and I heard her talking to her friends about how she only wants casual stuff out of relationships at the moment. I sometimes go bars with her group after our classes are finished and I’ve seen her chat up several different guys and leave for the night.

I know it’s absolutely none of my business and she’s free to do whatever (or whoever) she wants, but I don’t know why it’s weirding me out so much? Like I said I feel nothing towards her romantically or sexually so there’s no jealousy involved. I guess it’s kinda weird jumping from knowing her in elementary school to this?
Why am I so weirded out knowing about my childhood friends sex life?
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