Isaiah 58:9-14; Psalm 85(86):1-6; Luke 5:27-32
Jesus is clearly giving The Law short shrift - tax collectors are sinners because they associate with the Romans who are unclean because they are not Jews - Prostitutes clearly and publicly are sexual sinners (but what of those who (ab)use them??). The Pharisees are shocked by his behaviour - in their world view mercy eating with sinners makes yourself as bad as them.
The contrast that this story (and also those of Zachaeus, and Levi (Matthew) told elsewhere) is that the person is more important than the sin they have committed. In this day and age there may well be a tendency to avoid human contact with those who are public sinners. Certainly some sorts of sin attract a kind of inverse celebrity, and we all want to know who did what to whom and when. Doing what Jesus does is much harder - to actually give us his time to sit with and talk to. For we are all sinners, publicly or otherwise, and Jesus' actions with these notorious sinners of his time, should be a reassurance to us that we are all called to the table to dine with him.