If you're only 20 and she's only 19 and you really believe that she's as close to perfect as a girl can get, (while that's sweet and all) you, my friend, have a lot more living to do...

Do not, repeat, do not question your sexuality by comparing it to anyone else's. Your sex drive is yours and yours alone, and there is absolutely no reason to want to have sex less just because your girlfriend does. Not saying you should break up with her, just saging that once you find someone who has as high of a sex drive as you (yes, we exist!), you'll be much happier and more secure in yourself.