Help me decipher this.

So I'm moving into a new house in a month.

The Roommates:

1) Girl who I'm really good friends with but haven't fucked and have barely any desire to. She's pretty hot, a bit of an attention whore (fishes for compliments, etc.) but all in all a solid girl to have around.

2) Girl #1's best friend. She showed me a picture of her, and she's gorgeous. I told #1 that her friend is hot and pretty much laid it out that if I get the chance I will for sure fuck her. We seem to have a lot in common from what I know about her so far, so I'm interested.

So I'm constantly teasing girl #1 (my good friend) about how hot her friend is and stuff, and of course as she does, she marks her territory so to speak and says "You can't fuck her!"

This made me laugh, but I'm more interested in the meaning behind it and why she would say that.

Is there more to this, or am I just reading into this too much?