Why doesn't my girlfriend want my to buy a fleshlight?

Today, I told my girlfriend I want to buy a fleshlight. If you don't know what that is, it's a fake p*ssy that a guy masterbastes with. It feels and looks like a real one.She said it feels like I'm cheating on her because I'm "f*cking" another p*ssy. But what the hell, it's like, plastic. I don't get it, it's just a toy.