Is it normal i wanna see my gf getting banged??

Ok so.... im 23, and i have this odd fantasy about watching another guy have sex with my gf while she looks right at me and moans.... and i masterbate.. then she lets him finsih, and i take over and finsih myself... We have actually been talking about it and she said after ten minutes that its up to me.. she'd only do it for me, to turn me on, but she's ok with it.. but what makes us neverous is, would it ruin us? bacuase we dont want that to happen, we just want a wild night of dirty sex.. we are only 23 and have never done anythin like this... but im curious? what i wanna know is, has anyone done this and did it ruin you? whats it like, and is it really worth risking it?