I would sure hope that she would use a condom! At the very least you can be rest assured its only her you are tasting.
Sorry for the radical honesty over this one folks, but seriously safe sex should always be radically honest!
I can relate entirely to how you feel though. I think any of us can when we think of the first time we faced our partners heading out to be intimate with another. I know it pains mono deeply to think of the possibility of my being penetrated by anyone other than the three of my men. Sounds silly as there are three, but really its the unknown that is feared.
I remember my husbands first responses to my having been intimate with another man. It really helped that he knew him and had started a friendship with him beforehand. Very important to it feeling okay and acceptable.
I also remember the first time he was intimate with a woman he loved. I was extremely uncomfortable with the whole thing because she saw no value in knowing me or bothering to establish a relationship with me of any kind. I was sick to my stomach at the thought of his penis entering her and his mouth on her cunt. Blah! I still am that it even happened under those circumstances!
Needless to say, the relationship ended because of her inability to reach any kind of depth beyond sex and because of her belief that it was not important to know me.
Now my husband is seeing someone else and has my blessing. This man is respectful of our relationship and of me. He values knowing me which is important as it has become a prerequisite in his being intimate with another.
The man actually came over to me at an event we were both at and thanked me for my being open to sharing my man with him. I told him I was glad to because he is an amazing man and deserves amazing people in his life. That I was glad to share him with the right people. He is most definitely "redpepper approved!" (manners and consideration for others are so important!)
Don't forget that you have the right to make some guidelines. I always say that success in poly is based on the comfort of the most fearful. If you are not comfortable then it will not work.
You can also have the "vito" power if you both agree to it. I used it on my husband's last love. You can also find someone together for her. That way you can be more in control of who she reaches out to in pursuing another relationship.
All in good time my friend. There is no rush it seems and for that I would be grateful. You are in a good position to make this "your" journey together and that in itself is a gift. Some of us have it forced upon us.
One last thing, perhaps talking about what she misses about men and who she finds attractive would be helpful at some point too. When you can stomach it of course. Baby steps