I still fail to see the relevance. Their appearance is irrelevant. If they keep your house clean then it doesn't matter. If you're implying street kids won't do that, not only would you have to provide evidence that you won't be able to find, but it's also a pointless value judgement. I won't even get into the fact that Airbnb can often be cheaper, and so poor, savy people may use it and your behavior is putting them at a disadvantage basically because you don't like how they look like. Sound familiar?
People who don't "look filthy" could just as easily tear your house apart. Pointless discrimination is well, pointless.
It's not a "pointless value judgement" to observe that someone looks filthy, and to extend that observation to "I don't want them in my clean house".
Have you ever hung out with street kids? I have, for years. Do you know Manitou Springs in Colorado? I hosted lots of down-on-their-luck street kids at my apartment for a while until I realized the majority of them absolutely suck as roommates/tenants/whatever you want to call them. Many of them will steal from you because they have nothing else. Many of them have markedly different cultural values - and not in a way that I find endearing or enlightening.
I'm sure you'll draw some parallels to racism here, because that seems to be the argument you want to occur. This isn't about a genetic condition like skin color. This is about willingly descending to the lower rungs of society. If a street kid wants to drop out of society, tattoo their face, and wear tattered army rags, that's fine. Accept the consequences of your facial tattoos though - that's part of being an adult :)
I don't want people in my home who are proud of their guns and brandish them in profile pictures on a travel site.
I don't want people in my home who look filthy. I know street kids well enough to never let one in my home.