My boyfriend has no sex drive...advice?

My boyfriend and I have been together for several years, and we are in a good place. However, he doesn't really have a sex drive. I'm 29, and he's 21 (you would think it would be reversed!) In any case, its really frustrating because I do want to have sex with him, but he sometimes simply dismisses it or flat-out says he's not interested. I mean, we still do have sex - like once a week at best. I am, quite frankly, horny all the time and feel frustrated.Is it wrong that I am feeling sexually frustrated, and still am looking up porn regularly? I mean, sometimes I do think about sex with other guys (not that I really want to anyway) but because it could be an instant satisfaction thing (i.e., a hookup). I'd never cheat on him because I do love him, but is it entirely wrong to think these things?I just need help to figure out how to not feel so sexually distant from him.