I was in a similar situation recently and wanted to end a FWB scenario I was in. This is exactly what I said to her: "Babe, although I like what you and I have got going, it's turned us both into sexual objects. I don't think it's the right thing for us to do anymore" I said it with a straight face, and it worked perfectly. Women love sincerity in a man, if you can fake that, you've got it made.