Yes your in a tough situation and I have to say are really being a wonderful woman by trying to stand by his side and work things out the way you are.
I think you have been more than patient and it's time for him to be a man and face this problem with you head on, that is only way it will ever get fixed. Trying to be sensitive or nice about to not offend his man hood in anyway is really sweet and great of you, but he obviously is not getting the message that way.
It's time to confront this thing head on, you are about to marry this guy and live like this for the rest of your life,. Now if you are really into having sex as alot of poeple are and your fiance just has not interest in it, this could be a major problem for you and it can turn your marriage into a living hell.
You are right and a good woman for standing by your man and doing things the way you have been, but t sounds like he has no interest in doing anything about for you or showing any kind of understanding for the way you feel.
I wonder is it possible that he takes any kind of meds such as anti depressants or pain medication (narcotic) these meds can have a huge impact on a mans sexual desire and his testosterone level.?
And even if he does not take meds he may suffer from Low testosterone anyways. He should get checked for it and if he does indeed suffer from Low T, some T therapy (Which there many helpful therapy's), Such as the Gel or a patch or even shots, can boost his T level and give him back his erection and desire for sex.
Low Testosterone is a common probelm among men, more common than people even know. And young men who usually have low sex drive have Low T. So regardless of his age (And it sounds like you guys are fairly young) it is still possible and not uncommon to have Low T for no reason. Again if he is taking meds than Low T is even more likely depending on the med he takes. But its a simple blood test, and once performed the T test along with other hormones they check can reveal alot about his situation
Beleive me I'm now on T therapy because I have low T because of pain medication that I have taken for years and there are days were my sex drive is almost that of how it was when I was a teenager. (Due to being on Therapy for Low T)
But in order to disuss the above idea you guys are going to have to confront this together with "Open Dialogue", no more being afraid to discuss it. And just tell him it's nothing for you to feel bad about with me honey and that you are there for him and the reason you want to resolve this is so you can share your love for him more intimately. And he should have you involved with him with his doctor appointments and treatments if necessary.
Again if he does not want to face this and acknowledge your feelings on this matter I would seriously consider going into a marriage with him, unless you can accept the kind of life your living now for the rest of your life. There is nothing wrong for wanting to be sexually fullfilled and nothing for you to feel guilty about for expecting that from your husband. Even bilblically a man and a woman are expected to meet there partner needs sexually.
Good luck and I really wish you the best.
Let us know if there is anything else.