The Church of Jesus Christ of Latter-day Saints teaches that all sexual relationships outside of marriage defined as the legal union between a man and a woman are sinful. If your date has been an active member of the Church who is following these moral guidelines, then she will not be willing to involve herself in a sexual relationship with you.
If she is still Mormon and you are not, she will always secretly hope that you convert, just like you will always secretly hope she leaves the church. This is something that's very important to her, and it's difficult to be married to someone who doesn't share the same values.
Many blessings to you. I am a non-Mormon woman married to a Mormon manвwhich seems to be a less common scenario in the LDS world.