You could tell her that you once had a friend that had pictures or films of that, formerly, you had footage about people peeing and even (make a strange face) shitting on each other. Try to see her reaction. Most girls will be grossed out. So if that is the case, say boldly that you liked those kind of vids or pictures somehow, but you would never do something like that in reality. It is just a fantasy.
Tell her that having this litte perverted fantasy makes you feel a bit ashamed. If she feels sorry for you, the better. If, however, she does not feel sorry for you at least a little, become offensive. Tell her that having those fantasies also has good sides to it. She might ask why. Tell her that:
1. People into those fetishes are much more trustworthy and faithful. In fact, if you and your girlfriend share this fantasy, then this could create a stronger bond between each other than right now, because it is much harder to find a new girlfriend or become unfaithful than if you were just into normal stuff.
2. Girls pooping on you is not that terrible anyway. In fact, there are much more sick things in the world like that (I don’t wanna go into it right now)
3. If you go to the toilet, after you have done your pooping, do you run out and stick your head into the first bucket you can find? Of course not, so why would dropping a load onto someone be any different? It is all in your head. Because you think doing it into the toilet is right, and elsewhere is morally wrong?
4. Don’t you wanna become kinky sometimes?
5. Maybe the girls who do that kind of stuff have had some alcohol before…
6. Be glad I saw these kind of movies and pictures, because that way I am very open in bed. Or do you want to have a boyfriend that is only doing vanilla love, and never goes oral and never fulfills all your fantasies?
So those are a couple of things you could say. You could put reason Number 1 in last place for special effect.
If your partner is still totally against it (don’t force anything), then just quit talking about it for that day and just say: Yes, there are strange people out there, luckily I am not into that and I just saw the pictures and other stuff. But if she is just a bit playful, she might say, ok show me some of those videos. Do you still have them? In that case, show her harmless shit videos, not extreme stuff at the beginning.