You seem to be of the opinion that sex, any sex with consenting adults is perfectly acceptable. I say that for a specific reason and it's not because of hatred for anybody. You're asking who's threatened by same-sex marriage yet you consistently run away from questions that are merely taking your worldview to its logical conclusion.
Why stop at a marraige of two? Why not three, four, five? Why not between brothers? Sisters? Father and son(s)? Fathers(s) and son? Father and daughter? Grandparents and grandchild? As long as they're adults, why not? Is the American Family (your term) ready for that? Are you? Is that a good environment to promote and celebrate for children?
Scripter, I have the same answer for you as my reply to Squid.
What, ultimately, is the reason you raise these nightmare scenarios? Is it:
(1) you just want assurance that marriage will NOT be permitted for those specified nightmares?
If so---laws could be written to exclude those options. But would you then be OK with same-sex marriage? From everything you have written thus far, the answer is a absolutely NO!
So why even bring up those extreme scenarios and expend energy debating them? It just confuses the basic issue if you have no intention of saying yes to same-sex marriages under any circumstances!
(2) Consequently, I interpret your questions to be merely a debating strategy to raise the spectre of unimaginable horror so as to permanently preclude consideration of any less-scary and more typical options---by conflating the two. In other words, the slippery slope argument.
(3) Incidentally, have you done much research into how France, Netherlands, and Denmark have dealt with the nightmare scenarios in their countries? Has there been an explosion of fathers marrying sons, siblings demanding to be married, grandparents and grand-children etc.?
For that matter, in our own country, is there evidence that domestic partnerships have resulted in the odd couplings that you have proposed as being a likely end result?
Nightmare scenarios is a good description. As I've said many times now, it's merely taking your worldview to it's logical conclusion. Either you don't want to recognize where your worldview leads or your worldview prevents you from realizing where it leads. If you think that's a nightmare then perhaps you should rethink your position.
You said earlier in the thread that same-sex marriage was between two consenting adults. Well, what about three or more? They are consenting adults. Why not siblings and cross generational family members? If they are all consenting adults, why is it a nightmare scenario?
Who is harmed by this? Try asking former homosexuals. Ask former homosexuals to describe their former lifestyle. Richard Weller thought he was born gay. And the 12 years he lived the homosexual lifestyle he was 100% gay - he completely bought the lie. Now he is 100% recovered from, in his own words a "sick and twisted life."
Richard is one of the tens of thousands of former homosexuals.
To whom are these nightmare scenarios a threat?