Would worrying be irrational?

Hello,

I’m in my late teens and I want to start having sex. My boyfriend put on a condom and started to insert his penis about half way into my vagina, I got scared and told him to stop so he pulled right out. The condom was still completely intact. I should be old enough to know this may not be exactly how pregnancy works but I’m still anxious about it and I’m not sure why. I guess I would like some reassurance. Thank you.