Nemi, I was quite intimidated by penises when I was a virgin, and even for a short while after losing my virginity. I came from a relatively conservative background, so when my bf and I started having sex, I would glance at his penis and try not to think about how small my hole was and how large he was compared to the biggest tampon I'd ever used!
But after more exploration, sex, and general relaxation about the whole situation, I realized that my vagina was pretty flexible and that I actually enjoyed having his big cock filling me up. I got more into oral sex, too. So the penis became my friend.
Give yourself time to get adjusted to your new situation, and above all make sure you communicate with him about how you're feeling. I don't like him telling you there's no "natural response;" that's BS. Just because you're a little intimidated by his penis doesn't mean you won't love to have it in you... he should take a step back and let you choose the pace of your physical relationship. How did it make you feel when he said that? Make sure you tell him so that at least he knows where you're coming from.