My girlfriend wants to borrow my earphones, but I'm not going to let it happen.
I wouldn't either, but do you have a specific reason for not loaning her your earphones?
I just feel that I shouldn't have to share something that is mine.
Sharing is caring but in this case you are right. It is not good to share something that goes in your ear because they are not the cleanest space.
It's not like she wants to clean them.
I'd stick to my guns on this one and perhaps help your girlfriend research earphones that she can buy and have for her own use. This will be convenient, and clean, for both of you.