Why on earth would the defense bring up the "swingers" angle?
The case being tried is the murder of Danielle van Dam, not any criminal charges against the van Dams.Think about it for a second, even if you leave your car unlocked, with the keys in the ignition, the person who takes it gets arrested for theft.
Even if the van Dams are found to be negligent by not checking on Danielle, or even for inviting Westerfield into the house, the killer is still 100% responsible for the murder.
Even if the van Dams are found to be criminally negligent, that would be a different case, and a different trial.