I think the dramatic and permanent cutting-off of the family was Joe's idea: "But Danny, they
COVERED UP for a
DANGEROUS PEDOPHILE, you simply
can't allow this kind of pure sadistic evil into the beautiful kweer life we're trying to build together!"
I think Joe rightly sensed that the family did not approve of him. He felt threatened by them. When he saw a chance to drive a wedge between them, he took it, probably implying to Mallory that he would call off the wedding and leave her if she continued a relationship with these dangerous pedophile enablers. He made it even more dramatic by (as he admits) immediately bringing all their friends into the situation like it was some big emergency. Mallory is naive, weak-willed, and suggestible, so it worked.
Situations exactly like this (manipulator cuts off partner from family and friends) happen all the time.
EDIT: I had wanted to repost Joe's anti-breeder tweet but couldn't find it.
Here it is.
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