Things can't continue this way - of course you don't want to be stuck in a sexless relationship. And just as importantly, you shouldn't have to put up with a boyfriend who refuses to communicate with you about issues of vital importance, such as desire and intimacy.
While your boyfriend refuses to discuss this matter, I see no way of solving this problem, as you're left utterly in the dark over the root cause of his meagre libido (Does he lack feeling for you? Is he suffering from some form of sexual dysfunction? Has he been traumatised by a bad sexual experience in his past? Is he under severe stress, for instance at work? Does he secretly associate sex with dirty or immoral behaviour?).
So you must continue to encourage him to open up. Be gentle but firm - don't nag him or humiliate him into discussing his reluctance to have sex, but don't either leave him with the impression that he's free to keep his motives to himself. If you, or both of you, feel out of your depth in resolving this incredibly important yet highly sensitive issue, I urge you to seek the advice of a sex or relationship counsellor.
For a list of trained therapists, contact Relate, the counselling service for adults with relationship (and sexual) problems (Tel 0845-4561310 or check out: www.relate.org.uk). Good luck - I sincerely hope there's a happy ending to this difficult and touchy situation.