Sean, you have received such good advice from men who have in fact done what you are thinking of doing. It seems clear that you need to know the person well, already have a certain bond, you have to be comfortable with yourself, and she has to be someone you can trust cares for you to a good degree.
I have no experience in this at all.
So, what I would do is to try to think about the young lady and what questions she might have once you tell her:
1. Do you have an STD from this?
2. Are you going to become sexually abusive towards her?
3. Do you think you will want to have sex with a man or even a boy sometime? (very false myth)
4. Are you going to have good sex with her.
5. If you got married, would any children you have be safe?
6. Are you pretty much mentally ok now?
Maybe, a woman would not think like that. They are questions that I would want a young lady that I might love to ask her boyfriend, if they were feeling in love with each other and either were, or were thinking of, having intercourse.
Knowing answers you could give to such questions, will give you more self-confidence should you chose to tell her.
Several men here, and in other posts, have said that when the woman they love learns about the SA they act with great love and a certain protectiveness. I am not sure if you would want her to feel she wants to mother you or not. It could be kind of neat to a degree I would think.
I also have found that in telling a story to someone other than your therapist, a bare bones kind of explanation is all that was needed. "I was sexually molested as a kid--one man did it, about X time(s). It invovled both oral and anal sex." Why would one need to know more? I can't see that they would. Even that might be more than is needed at first. I would think that if you could talk about the news about boys being molested, and in this simply say that, that had happened to you, might be all someone can take who loves you.
Sean, I have boys for whom I was their spiritual mentor for three or four years. They were abused while studying at a high school I taught at. When I first heard of that I went into a profound sadness, I cried a lot and felt so helpless to be of any help to them. Eventually, several of them came to me, now several years after the abuse and talked to me about it. Sometimes, they told me some details of the abuse.
I listened. I think I acted appropriately. We cried, we got angry about it. We talked about how they were now. Then we mapped out a way for them to move along towards healing and feeling better about themselves.
There is still a part of me that hurts about that. I am happy that we can hug and they want me to hold them some. But, although I loved them as young men, I love and respect them all the more today. And incidentally, all of them are very happy and successful men. Some are married and have children. One is in a very loving relationship with his partner, that looks like it could be permanent.
My hunch is, that in most cases, if the girl you are speaking about really has come to care a lot about you--loves you, she will hurt for you, be angry at the abuser but most of all she will love you even more.
It is sad though, that such is not universal. Some men tell and the relationship gets strained and breaks.
Hopefully, the men here have given you what you need to decide IF you want to tell her, and if so, WHEN, and HOW.
Just remember Sean, no matter the reaction, you are a good man, a strong and couragesous man, a total man. You are a better person than men who have never had a really traumatic thing happen in their lives. Don't let a response that is hurtful at the time, lead you to think less of yourself or to see yourself as in some way a "damaged man." You are not damaged goods.
Proud to be your brother,
Bob