I think in the film they had made out that Aragorn might think Arwen to have somehow 'passed away'. There is the scene where Elrond tells Aragorn that Arwen is dying (the one where Elrond turns up at Dunharrow to indluge his love for cloak swirling in a camp fashion...
), and then couple this with the scene where the Evenstar 'breaks', and Aragorn may have lost some of his hope that Arwen would be his. So in the circumstances, his passion is quite understandable!
I didn't see anything untoward in their previous scenes together; I think these were left quite 'open' in the sense that the viewer could read into them what he or she wanted to see. Though looking at such scenes in the context of the film version of the story, the fact that Arwen has a vision of the child she would go on to have with Aragorn might also suggest that they had not yet slept together.
It is important to view the whole romance as something quite different to the one portrayed in the books. Tolkien stated that Elves did not have marriage ceremonies and that the 'act of union', as I think he delicately puts it, stood for such a ceremony and created a permanent bond - so as seen in the films, if they
had slept together then they would have been
already married; I think this shows why it's important to separate film and book. As to whether all Elves were always happy with their chosen partner, there is one example of a union that ended in a break - and that is when *Finwe's first 'wife' wearies of her physical existence and he takes another 'wife', but the eventual results of this are not
entirely happy.
*EDIT - D'oh! I had originally put
Feanor's first wife! A large handshake to anyone who managed to spot it before this swift edit.
