"Josh wasn't aware of a lot of these things at first because the first two or three years of his marriage -- it was really pretty much not something that really came to light," Steven Powell told ABC News. "We interacted in a lot of sexual ways because Susan enjoys doing that."
Steven Powell plans to post his daughter-in-law's teenage journals on the website SusanPowell.org; according to the Salt Lake Tribune, he says that the journals show that his daughter-in-law is capable of walking out on her husband and their two sons.
He has publicly theorized that she disappeared with a man from St. George, Utah, who disappeared from a Christmas party abruptly the same night she vanished.
Susan Powell's parents, Chuck and Judy Cox, maintain their daughter was a stable person -- and feel Josh Powell should be doing more to help police solve this mystery.
"I see Josh as a desperate person," Chuck Cox told ABC News. "He needs to find some way to excuse his behavior or deflect attention, and he is unwilling to cooperate with the police department.
"I do not believe that he went camping in the middle of the night with two young boys. I know Susan wouldn't let him do it," he said.
ABC News' Lauren Effron contributed to this report.