Why does the idea of masturbation make me so uncomfortable?

I’m 18F and I’ve never tried masturbating before. The idea of me doing that makes me uncomfortable and I don’t know why. I’m atheist and religion is not a part of my daily life or decision making, and I don’t have super strict parents who would be against it, so please don’t include that in your response...

I was at a sleepover and my best friends of a decade and I were talking about sex and eventually masturbation. Only one of us had ever had sex, so for some reason I just assumed that the rest of us were all self-virgins too lol. They were all super weirded out that I had never even tried and said I was missing out. They also emphasized how odd it was that I found it weird to do so, and that I should assume that everyone does it instead of not do it.

I have a vivid imagination and I still haven’t gotten the intrusive thoughts about my best friends touching themselves— it kind of opened my eyes to a side I never knew they had and makes me annoyingly uncomfortable because I know it’s normal.

But then it’s just confusing now, because I find the concept of someone attractive touching themselves as sexually attractive. Like, I’m definitely not asexual. Just the idea of people I’m close to doing that is gross, and even more the idea of me doing that. My hunch is that it’s something about loss of control or vulnerability which has always made me unreasonably tense.

What do you think is the reason behind this weird feeling, and how should I proceed with my thoughts about my friends and myself?