Mama June -- Sugar Bear Always Took Back Seat to Child Molester Mark McDaniel

Child molester Mark McDaniel was the Camilla Parker-Bowles in the relationship between Mama June and Sugar Bear ... and even though he was behind bars almost the entire time June was with Sugar Bear, it destroyed their relationship.

Multiple family sources tell TMZ ... June kept a box of photos of her and McDaniel in the house ... something Sugar Bear knew about and protested, to no avail. She would NOT get rid of them.

We're told McDaniel -- who molested June's then 8-year-old daughter Anna -- was the subject of frequent arguments between June and Sugar Bear. He felt she was still emotionally in a relationship with McDaniel ... even though he had been locked up for years.

Our sources say June was seeing McDaniel while she was still with Sugar Bear -- he got out of prison in March. The family now believes it was actually June who was cheating, not Sugar Bear, and she used the online dating story to get out of a relationship she had already rejected.

Sugar Bear is Honey Boo Boo's dad, and we're guessing he's not happy about the pic of McDaniel touching her ... a pic taken Sept. 9.