No

"Carrie. You have just admitted that you love your sister. Hurting her means hurting you. And I love you too much to allow this to happen" You say after throwing away all mental images of using Cassidy in various ways. "I just can't do anything that can hurt you"

Your girlfriend melts away in a somewhat silly smile "I love you, too. I always knew that you are a perfect guy for me. But still," her facial expression changes to a sad one "I want you to fulfill all of your fantasies. To live out any of your dreams"

"Some are actually impossible to fulfill, Carrie." At least she doesn't insist. Her willingness to assist you in **** her sister was kinda scary.

"What if I'll find some other girl that looks like me? Will you **** her?" Looks like your relief is premature. She hasn't given up this idea "There are millions and millions of woman, someone must look like me."

"Carrie..." She interrupts you before you actually manage to object and say that you aren't planning to **** anyone "and why one who looks like me? It is not like I'll be jealous or something should you **** a random woman" Carrie speaks with a voice full of enthusiasm "Why not something that will give you a new experience? Latino girl, or black, or Asian. Yes. Yes. Yes. You need to **** someone just to learn if you actually like it. And if you do... Imagine us living a life of serial rapists? Traveling from city to city hunting for girls. That would be so great!"

This is becoming very weird in an unpleasant way. You are almost tempted to say that she is crazy but ruining relationships (and losing all great sex) in this way doesn't seem like a great idea.