First lets just acknowledge that the "friend" is you.
I'm not sure if you understand how it might sound really odd, desperate and shallow to have sex with someone that only has ONE single asset you find attractive on their whole body??
Are you just planning on using the girl with the killer tits to get your rocks off?
I wonder how and why you even bothered getting into her pants at all if she's so unnatractive to you.
Do you have a fetish for tits or do you just see her as an easy option? Maybe she's the only person you can manage to find to have sex with ... what does that say about you?
I assume she must really like you if you're allowed to get it from her repeatedly. I wonder what attracts her to you though.
How do you think she'd feel if she knew you only wanted sex with her because she has attractive tits but nothing else.
If you can only find one thing about her that you like, how does that justify having more sex with her? That's a bit of a shit go.