As long as it wasn't breaking the law yes I would respect whatever decision the parents gave. Even allowing her to have sexual relations with someone else.

What if Abby's parents routinely bullied her and committed acts of emotional abuse towards her, like regularly telling her that she is worthless, that she will never amount to anything, and that everything she enjoys in life is "stupid"? Would you have no problem with this whatsoever as their "right" due to the strictures of parental sovereignty?

I'm not asking if you would intervene or not, as I think we both acknowledge the legal fact that they have more or less absolute power over Abby - with their only rival in this area perhaps being the state - which means your intervention would come to naught save perhaps your quick ejection from their life, or at least told to "mind your own business." I'm simply asking if you wish the community, including yourself, had the ability to act when a youth under a certain age were terribly unhappy and bullied under the custody (i.e., control) of their parents. Would that matter to you at all if such routinely occurred in your field of vision?

In other words, what is more important in the end if the two come into conflict - Abby's happiness and emotional well-being, or the authority of her parents?

The above scenario is not rare, and happens to all too many kids, with the parents' actions either tolerated or rationalized as a legitimate "method of parenting."

These, I think, are important questions you should ask yourself as part of your quest to expand your learning horizons and provide yourself with nourishing food for thought.