My ex and I were nearly a foot apart - we were married for a long time (21 years) but I wouldn't say it was the best relationship. I would guess this is one those odd correlations that aren't actually causation.
My DH was just under 6 ft (he's shrunk a little) & I'm just over 5 ft. We've been together for 33 yrs & married for 32. We're still good.
I'm going to make sure that I boast proudly about giving Jamie 14" for the past seventeen years.
DH & I are 15" apart. We look ridiculous in pictures together, but I kinda like him a lot... So I'll keep him.
A fraternity brother in college is 7'1". He married his high school sweetheart who is 5'. He wins.
(And, yes, he played basketball and people stare in restaurants.)