It was a big company. You could date and marry coworkers but you couldn't date someone in your chain of command.
I didn't know the internal structure of the IT department, but if he was high enough in the chain of command, there would be many positions below him. I had an entry level job. Transferring from an entry level job in a different department would have meant I would be getting an entry level job in IT.
I was having trouble figuring out how to get a different job in the company as is. I was having trouble finding the kind of info I wanted that was pertinent to me and having trouble understanding the internal job listings.
Adding the possibility that someone had just asked me out who was high enough in the department I wanted to get into that many of the jobs that might interest me would make him my boss made it overwhelmingly difficult to try to navigate the process of transferring into IT.
As I said, I already had plans to leave for unrelated reasons. Had I stayed, maybe I would have eventually drawn different conclusions and found a path forward.
But based on the info I had, my emotional reaction was "Welp, I can stop fretting about whether or not I'm doing the right thing by leaving. I'm basically going nowhere fast at this company."