Comparing penis sizes of partners ?

Comparison is an intelligence trait. We contrast everyone we know based on how they measure against other people. She will without a doubt be aware of the differences between your penis and her Ex's and any other penis she's seen in her life. And without a doubt she will care less about this than you do.

Think about how your girlfriend compares to every other girl you've been in love with. Then think about how few of the things you're comparing have to do with sex versus her manners or kindness or intelligence. She's not with you because of the dimensions of a piece of your anatomy she hasn't seen yet and once she sees it, it still won't be a deciding factor in what she thinks about you.

Also never be mad at someone for giving you information. She has told you this doesn't matter to her. Any problem you have with her Ex's anaotmy is entirely on you for being insecure about your abilities.

I know, I've told her that it's natural to compare and she still denies that she would.

She's actually my first girlfriend, I've never done anything with other girls either, I'm her fifth boyfriend and she's done everything but had sex with all of them.

What does it matter if she compares you to another man?
Why would it matter if she's done things with other boys?