I expect that discussing it makes him feel shame and emasculated and so he avoids it. You might try going alone to a sex therapist and explaining the situation. The T, i expect, will be supportive and say that there is treatment available. You vould convey that information to your fiance and perhaps he would then be willing to see the T. I expect that he might want to speak to the t alone for a session or two and i think the t will do that.

Of course you would be going over all of this with him in a kind way, but also (i suggest) in a firm way. If he absolutely refuses to get help, you might then ask yourself if you really want to marry a guy who not only may be unable to share sexuality with you, but more importantly a guy who refuses to face and discuss problems with you.