This was my take on it:
Their differences were blatant and strong, but still exciting. Great conversations

. Their outcome together might have been better had tenderness
been a part of the basis of their relationship [their relationship being from a distance? somehow, this reminds me of an Internet-borne relationship], but at least passion they did have and they enjoyed interacting in that way, and hoped/presumed the tenderness would follow once they were together.
However, with so much
emphasis on the passion, once they were actually together [met in person?], they found their relationship
lacked that same passion and their relationship faded 'gently' into the long, dark night... with the passion absent, their relationship failed.
So, instead of the relationship
thriving based on tenderness and gentleness; it instead
died, earmarked by those qualities.
Had the relationship been based on/built upon the tenderness and gentleness... its remaining that would have been fine. Had the initial passion [borne out of their differences] been sustained, the relationship might have survived. However, it seems the qualities/dynamics of their relationship ['before/after'] were like ships crossing in the night... at cross purposes with each other.
Boy. That took a lot.
Now, William, you can just dismiss it all with... "Thanks for those thoughts, but you're wrong."
~ Lizzy
With my interpretation, however, I wonder exactly what is the gift. Was it the initial passion that is now gone? I guess, with reading Manna's and my interpretations, and still being unclear on the answer to that [as hers would seem to suggest that it is the gentleness that is the gift... but then why does the word "failure" get used]... anyway, the bottom line is that I'm confused as to what is being said in this poem. "Biding" is a word I generally associate with "time," as in "biding my time"... so is the relationship just going to play itself out in what's left of their time together, as a gentle, but unpassionate, 'failure' of a relationship... or is the passion 'failing' by morphing into gentleness actually the gift? Even though it might be said to have failed, in the presence of gentleness, is the passion not really that missed? I hope you'll clarify something here, William

.