I want to wipe my feet on my girlfriend's facial towel
That's disgusting! How could you do that?
Face towel is there for a reason. What's the difference, feet or face?
Towels are to be used as you need them to, but this was one intentionally made for the face only. Take the other towels for your feet, there is a reason they are multi-purpose towels. If you wipe your feet with her face towel she will have gross things on her face!
I've been using it for a while and she hasn't noticed before.
Try to show her some respect. Women have specific items so they don't cross contaminate or they have a special purpose. You wouldn't use sewing scissors to cut paper would you?