When Bob interviewed Todd and Lizette, his questions revealed some things about them they didn't know. And they wanted to explore this new knowledge. Naturally, since Bob had helped them identify some kinks in their lifestyle, he was invited to help them explore those kinks.
And then their college aged daughter got curious about Bob and who he was and just why the hell her mother would let this stranger see her almost naked.
So SHE wanted to explore things too!
They might have found out more than they intended ... except that everybody had a great time, and Bob was made a friend of the family for life.