Aragorn and Arwen: More than kissing?

[Lush]

In the films we see Arwen heading for the boat to Valinor, after the scene where they have supposedly slept together

I thought that was one of the most screwed-up plot changes in the entire three films combined. I think they simply added that for dramatic effect; I do not believe that their intention was to try to define the status of the Aragorn and Arwen relationship. Therefore, I have a hard time using that as evidence to back up either argument in regards to whether or not they were already "doing it."

our modern eyes read a lot more into some things so I do understand why it is read that way - I mean the mores re pre-marital sex have just about completely reversed - when Tolkien was a young man, couples were expected to wait and a nos jours .....

Yes, but if we are to agree that for Elves sex is an act of marriage, then there is no such thing as pre-marital sex. It's not even an issue. Hence, if Aragorn & Arwen, or Beren & Luthien, are "doing it," then that's their business. I looked back at my Luthien thread to refresh my memory on your comments there; and I agree that it throws new light on the issue, but I'm also not sure that Beren & Luthien went on that dangerous quest just to be able to, you know, have sex. I think Beren's reputation and Luthien's love for her father were at stake as much as anything. I would agree that having no public ceremony was not really done, but when you take the entire tale in context and all the other horrible stuff going on around Beren & Luthien at that time, it actually makes sense for them to keep their commitment private.

Could you possibly provide the HoME quote you are referring to?