In Utah, if you act in defense of a third party, you have all of the rights that they have. You also have all of the LIABILITIES that they have. Meaning, if you jump in and start killing people when you aren't ABSOLUTELY sure of the situation, you have a BIG problem.
I would be very, VERY careful about intervening in any situation I perceived as rape. Yell stop, pull out your phone, perhaps, depending on the situation put a hand on my weapon without pulling it, but I WOULD NOT pull and shoot immediately. WHY? Because there is a possibility one of two things will happen.
1. I pull and permanently stop the rapist's actions. The 'victim' pushes what is left of him off of her, and yells at me; "What are you doing?! We were playing a sex game and you killed him?!" Where I live we call this situation manslaughter.
2. I pull and shoot the bad guy. The police come and get statements from everyone. I tell the police that I saw a violent felony in progress and I acted to stop it. (Through my lawyer.) They ask her to confirm this, and she doesn't feel like talking. She doesn't want to go to the hospital to run a rape kit. All she wants to do is go home, take a bath, and sleep. She won't give a statement. So the police ask her friends, and they all say that the rapist was her boyfriend, and they had a sexual relationship for the past several months, and it seems very unlikely that he would rape her. Bottom line, the only one who says it was a rape is ME. SEE #1.
I have a VERY short list of friends and family members I would act do defend with deadly force. I have FAMILY MEMBERS I wouldn't act to help, because their lifestyle is such that I would not know for sure that they aren't an active party in whatever terrible thing that I am shooting to stop. The people I would act for are people I not only have a strong knowledge of their background and lifestyle, I care enough about them to go to prison for them if I'm wrong. This makes it a VERY narrow list indeed. Think about it.