Quote from Martinghoul:
Well, let me make a general point first. My approach to these things is fully libertarian. I don't see why actions performed by capable adults which don't harm other persons should be subject to regulation by some big central agency, be it the Church or the State. I don't see how any "small government" conservative could possibly call themselves that if they want this "small government" to be in the business of regulating and enforcing some code of sexual behaviour. Thomas Jefferson said it best: "a wise and frugal Government, which shall restrain men from injuring one another, shall leave them otherwise free to regulate their own pursuits of industry and improvement".
As to your specific cases, no, I don't have a problem with the first two cases, as long as the people in question can make their decisions freely. There is an issue with incest, in that it can cause the offspring of a couple you describe, who obviously can't consent, to suffer and to need the State's care. So, in the first case, I would demand that the parents are sterile before they enter into such a union.
As to the third case, the young woman in question is either capable of making her decisions freely or she isn't. If she is, I have no problem with it. If she isn't, I do.
And no, I am not forcing anyone to accept the cases you've given as normal. All I am asking is for you to respect people's individual freedoms to make decisions, regardless of how misguided these decisions might be. You (and I, for that matter) might think that these things are not normal, but those are just opinions.
The State shouldn't be in the business of protecting people from themselves or helping the Church do that. And no, this still doesn't tell me why my opinion is less valid than pspr's, 'cause I have no idea what endgame we're supposed to be seeing here.