A bar of soap is the only thing that people insist other people rub against their asses, only to lather up and wash their own face. Think about it, is there another object or item that you seriously want people to use and then take said object or item and to happily use on your face? No, there isn't... I just answered the question for you.
I sometimes have to tell my significant other to take a shower. Hey, they're busy and they like to be outside doing stuff and whatnot. Sometimes... sometimes, they start to stink. B.O. is a helluva drug, but it ain't one for me. So, when those moments arise and, say, this significant other wants to get freaky in the sack, I just might insist that they go and wash their nether-regions with a bar of soap.
That bar of soap, the one that washed an almighty stinky ass, the one that was embraced by dirty fingers and rubbed up and down, up and down against a crusty, stinky, mother-f'n filthy ass-butt, is the same bar of soap that I will HAPPILY use to wash my very own face. Isn't that weird? Yes, it is... I just answered the question for you.
The ONLY thing that voluntarily touches the ass of another human and then voluntarily touches the face of another human is a bar of soap.
Man, being human sorta sucks.