First, tell her that you are very careful about choosing partners because you are concerned about STDs. Women who feel the same way will be reassured. Then tell her that in this day and age, this is a real issue and you feel strongly that partners should openly discuss their sexual history and get tested to support their health. Make sure to say that you do not mean to judge her or be intrusive, but that you need to know what her situation has been for health reasons. (You're not making a moral assessment about her sex life.) If you share about yourself first in a loving way, most women will not be offended unless they're hiding something.