I can't believe I'm doing this...but here goes....

My girlfriend has lost interest in sex after only 5 months. We used to have sex a few times a week, now its down to every week to a week and a half. The thing that bugs me is she's told me with her exes she had sex alot more, up to 4 times a day. We love each other and everything else is good between us, and I'm not the kind of guy that's going to leave her over something like this, but I can't help thinking if its changed this fast, what's it going to be like in 20 years? I've asked her about this and she hasn't given me an answer, says she doesn't know. I hate to compare myself to her exes because they all treated her terribly, but part of me can't help it and this is starting to eat at me. Any help would be great.