Preston didn't just sleep with an elder - he repeatedly used Bex for sex and as a cover for his relationship with Michelle. I think that was the main reason Martin was so angry. It was difficult for me to feel sorry for Preston, I guess because they didn't want us to, but I don't know if Martin accidentally running over someone would stop him from being furious at his teenage daughter being used that way.
The pity parties for Michelle are nauseating, although I did like the scene where she wore Preston down until he agreed to leave, because they reminded me a lot of what Den would have done.