You're right in a way Lisa - it does 'sustain' the relationship.
But what 'relationship' are we talking about?
Are we talking about the 'Eve' relationship in regards to 'physical' contact (sex) and having children?
Or the 'Lilith' relationship in regards to 'mental' and creating something like what Dante found in Beatrice which resulted in the immortal "Divine Comedy"?
(I think Daniel Day-Lewis found the Lilith in Michelle Pfeiffer but Eve in Winona Ryder in a movie called "A Room with a View" ...i think?)
Dave Stewart and Annie Lennox with Eurythmics. Christopher Dean and Jane Torville with IceSkating.
If it were akin to the 'Lilithian', then long-distance is easily sustained and blossoms more-so thanks to the Internet.
...as for having a 'procreative' relationship in regards to kiddies, then long-distance ain't the go I guess.
I like this saying:
What do you get with a...
Smart Man & a Smart Woman = Romance
Smart Man & a Dumb Woman = Affair
Dumb Man & a Smart Woman = Marriage
Dumb Man & a Dumb Woman = Pregnant

...Some people can consider this the x4 stages in their Relationships while others just stick with the one in a state of being.
The best Romance is never a sexual one.