I have been thinking about this too. The many mismatched couples Jane presents to us.

There is Fanny and John where he is said to have turned out worse than his initial character necessitated because he married such a wife.
There is - as you already said - Sir John who is always restlessly looking for company, as if he abhors being alone in his splendid house with only his indifferent wife for company.
There are the disparate Palmers who don't seem to have any connection whatsoever apart from houses and a soon-to-be baby.
There is the horrid prospect of a match between clever, considerate and reserved Edward and the ingratiating and selfish intriguant Lucy Steel.
And there is the overheated romance between Willoughby and Marianne where at first they seem to be perfectly suited until he makes his abrupt escape. Her youth can perhaps serve as an excuse for unguarded behaviour but what is he about?
Then there is colonel Brandon who has obviously defied several attempts of Mrs Jenning's to link him to just any young woman. He seems to be the first sensible man about.