Why did my girlfriend want me to have sex, then we don't do it, and now she turns me off?

I don't get why I say things so STUPID, like **** up in front of her about something disgusting. Like for ex, my sister wanted me to DEFINITELY do it with her, but we KNOW THAT we're not related. But why though, is she saying after is what turned me off, that we are somehow 1/4 related which DISGUSTS ME, and then tell everybody that I'm trash for f*cking her. Now, I can't f*ck anyone ever again. Because and know that I would NEVER DO IT with my real sister, and then she turns me off about how we could never **** again. And this seperation bullshit, like has to do with incest on **** I've never done. Wtf? Thanks to her, my girlfriend will never talk to me now. And I just recently had a mental breakdown.