Don't Misread the Ending (Spoilers)
While the ending is ambiguous, it doesn't require the viewer to assume the relationship did or did not come to an immediate ending. I'd argue that it was Anna who did the most to keep the relationship going - she was willing to relocate to LA even with her budding career, the responsibility of settling the visa mess was hers, and she initiated the marriage discussion. Therefore, when we see that it's she who leaves the shower at the end, we know something's up - some (or all) of the passion has gone.
Might she have gotten dressed immediately after the shower and driven to the airport to go back to London? Probably not, she didn't have her job and she had fought so long to get back to Jacob, that even if the relationship was truly dead by the time they shared the shower, they were going to give it a futile try.
There's an argument too that the attempt to keep the relationship wasn't futile. No relationship remains static - either they fizzle and die, or they evolve. When they evolve, passion gives way to other things that cement love in the relationship, such as the kindness and genuineness each had for the other (no depriving of bacon for Jacob and Scotch for Anna). They'd been through a lot, but it's plausible their love and compatibility could keep them together.
To me, the ending isn't to make me wonder what happens to the relationship, but to show that all loving relationships begin in a cocoon of passion, which cannot last forever. We see Anna and Jacob's cocoon break, which is what happens in the real world, and it's sad.