Is it normal to be sexually apathetic?

So, I'm eight-teen years old, male and am currently in the third year of a relationship and going strong. Thing is, over the course of the last year we've (Girlfriend and I) been sexually involved and I've noticed that I've never really been into it at all. Lately me and her have only been having sex about once every month and a half, and only because she wants to. What's more, I can't even remember the last time I masturbated or even looked at a girl with sexual intent. I feel 100% sexually apathetic; completely disinterested in anything sexual. It really wouldn't be a problem (I.e I don't really miss it) but my girlfriend is taking it personally and it's hurting her self esteem.