I know I didn't really experience real sexual arousal til a little later in my teen years then discovered I was bisexual. Before then I wasn't really interested, though I had some mild interest in both genders. Some people are late bloomers. That being said, you could be asexual. Most people I know who are asexual can still arouse themselves and masturbate, but others don't really want or need to. It's really whatever makes you happy. Also, don't be afraid to experiment to see what you really like and what you're really into. There's a lot of different things that can cause arousal other than the obvious stuff. Like I said though, it's whatever makes you happy.