Hmm, I wouldn't worry. Honestly, I'm going through the exact same thing with my relationship.
Him getting hard just means you're a turn on. In guy talk at least. Which is a good thing, because you're beautiful, and he gets that feeling whenever he sees you and that makes him horny. It would be a bad thing if you were a turn off, wouldn't it? Especially to your boyriend. With guys, doing sexual stuff is a way of showing that they love you. For most girls, they think of it in a way that they're being lusted after, and they only reason they're in a relationship is so they can have you as a little sex toy. That's really not the case with guys who think love = sex, which is just fine.
But if he wants more of a sexual relationship then a romantic relationship, maybe it's not the best thing to do. A more romantic relationship could be more helpful for the future, but it's natural to have a little bit of lust in it ^_^ Its better to love through emotions then physically in love =P