Being nervous about sex with a total stranger has nothing to do with when that person lost their virginity. There are many reasons to be concerned. First of all, you don't know them. You don't know that you can trust them to adhere to your boundaries. You don't know if they're telling the truth about sexual health history. You don't know if that person is going to hurt you. Though, it's the sexual health history that I'd be most worried about. I would rather know the person I'm about to sleep with. I would rather have a friendship with them first, if not a relationship. Then I'd be able to trust them.
But even first time with someone you know well can be a bit uncomfortable. You don't know yet how your sexual chemistry will be. You're not sure if you'll be able to read each other well and judge their reactions accurately. It's the unknown that makes people nervous.
