I had a long term boyfriend in law school who loved the idea of watching me with other men. He thought the idea was sexy because he would have enjoyed seeing other men get off on how sexy his girl was. To him, it was flattering. I didn't really want to go there, so it never happened.
I honestly think that you are going to have to apply the brakes to the idea of bringing in a male to your bed, because you are just not at all comfortable with it. Just because she agreed to bring in a girl, does not mean that you owe her to bring in a guy. Look at it this way, if she agreed to have anal sex with you, does that mean that you now "owe her" to allow her to put on a strap on and have anal sex with you? Hell no! She agreed to and enjoyed having the woman in bed, and you don't owe her a darn thing. You cruise down a dangerous highway when you start agreeing to bring other people in and you are not comfortable with it.
If I were her, I would be flattered and would love you more if you refused. I would want you to say that there is no way that you could watch a man with me, because you love me too much and it would break your heart. Believe me, if any woman is upset after you explain it to her like that, she is not in love with you and is just out to get as much sex as she can and should also probably have her head examined.