The respectable man sees Jesus allowing the not respectable woman to approach him, even to touch him. From this, he concludes that Jesus can't be a prophet if he can't even tell who's deserves to be allowed to get close.
Think about your own congregation. If respectable people judged you negatively based on the number of unrespectable people in your pews, what would they say about you? Jesus pointed out that the woman had performed actions which really should have been done--but weren't--by the host. Think about your congregation again. What are some ministries that you are neglecting to do? Could you use some outsiders to point them out to you?
Jesus responded to the criticism by posing a question, a not unfamiliar trait of his. "Who would be most grateful to have a debt forgiven--somebody who owed a lot or somebody who owed a little?"
The host was worried about being tainted. The uninvited guest was grateful for the gift that Jesus provides.