Megan has an AIDS-phobia (Whatever scientific term is) and it would be a plot point eventually
Her reaction to homeless lady was weird (She should know that AIDS does not transmit like this). Maybe it was irrational fear speaking? It would also explain her outspoken abstinence.
Both pairs revel in their poverty and the filth of their broken city (the parallel between the "lake" Joel and Gary create and the ruined pool Charlie and Mac try to repair is an example of their similar mindset). Also, Joel is hopelessly optimistic and gets incredibly angry at bizarre, insignificant things (like Charlie).