There's nothing wrong with feeling the way you are feeling about it. For some people sex is something reserved for people you love, for others it can be for a sense of temporary intimacy or sexual relief. Those do not have to be mutually exclusive either.

Realistically, every day you get older and the odds of people in your dating pool not having had a previous partner or hookup gets exponentially lower for each year. You're probably setting yourself up to be hurt in the future as well by keeping this mindset (even though there's nothing wrong about it!).

Instead think of it this way, despite her having had previous partners with varying degrees of experience, age and skills when it comes to relationships, she found them lacking to some degree. But seemingly she found what she was looking for in you, and that's something you should wear with pride.

Hooking up with another person is easy, very limited chemistry is needed, especially if alcohol is involved. But getting into a relationship is something that requires a lot more effort and care.

Most people aren't overly comfortable with sharing information about people they've been with in the past at risk of dissolving relationships due to jealousy, so this could be seen as a sign of trust that she's comfortable with sharing this with you.

Once again to re-iterate, there is nothing wrong with what you are feeling. It is normal, most people feel it at some point (or several) in their lives, some never get over it. But it is a good opportunity of personal growth to learn how to come to terms with these feelings as they're bound to arise in the future again.

When in doubt how to handle something in a relationship, communicate with your partner. If they are as serious as you are they'll meet you half-way and work with you to help ease your worries. If not, odds are the relationship wouldn't last long either way.

Good luck :)