It could be that she was shamed in her past or feels that being with another person or wanting to be with another person (either you or her) sexually is shameful (or even that sex itself is shameful, a real problem!). People who are shamed are very unwilling to talk about the thing that makes them feel shameful. I'm not thinking that you can undo this damage that has been done. Professional help may be what's needed on her part to help her see that what you are requesting is in many cases very natural and nothing to be ashamed of. Our upbringing is very hard to overcome, especially those messages of unworthiness and unlovableness. Your unconditional love is the only antidote that may help, that is, letting her know that whatever and however she feels, you will still love her. She may not believe you at first, but experience is the best proof. Good luck, this may be a really tough issue to overcome, but when you do, the whole relationship should improve!