Hi, I'd like to help you with your question.
This is about issues from her past. Your fiancee is traumatized from the abuse she suffered. Seeing male anatomy reminds her of what she went through and she has trouble facing how she feels so she is repulsed. This is not about you personally, just her feelings in general. It may also be why she finds being bi sexual appealing. To her, sex with a female is not traumatizing and most likely makes her feel comforted.
It sounds like your fiancee loves you. But by being with you, she is reminded of her trauma so she tries to work around it so she can be with you. This includes asking you to wear at underwear when you are near her so she does not have to face triggering her trauma.
Your fiancee may have PTSD from what she has gone through. Avoiding reminders of trauma is a symptom of PTSD. It is very common that survivors of abuse suffer from symptoms of PTSD, often for years. Is your fiancee in therapy? If not, she would benefit from seeing a therapist to help her overcome her trauma and be able to express her feelings with you. To find a therapist, she can ask her doctor for a referral. Or she can search on line at http://therapists.psychologytoday.com/rms/.
She can also work on her issues at home. There are numerous resources to help her. Here are some to get her started:
The Sexual Healing Journey: A Guide for Survivors of Sexual Abuse (Revised Edition) by Wendy Maltz
The Wounded Heart: Hope for Adult Victims of Childhood Sexual Abuse by Dan B Allender Ph.D. and Dan B. Allender
The Survivor's Guide to Sex: How to Have an Empowered Sex Life After Child Sexual Abuse by Staci Haines
You can find these books on Amazon.com or your local library may have them for you.
I hope this has helped you,