The Addams Family TV show: 1964-66
The Munsters TV show: 1964-66
First appearence of the Inhumans: 1965.
Looking back at it, I don't think it was a coincidence at all. This sort of melding of popular culture with the Marvel Universe was something that Lee and Kirby (especially Jack Kirby) excelled at a few years later. The Inhumans began appearing in late 1965 (not counting Medusa who was introduced without an origin more than a year earlier). By 1968 and 1969, Stan was openly promoting the take-offs of pop culture in the FF. Two of the most memorable were the 1930s gangster plot (an entire planet of Skrulls were so taken with gangster movies that they redid their planet like one) and the Prisoner plot (Doom imprisoned the FF in a village much like the one in the classic TV series).
With this in mind, you can almost see the story conference. One of them (I'm going to guess Kirby) suggested that they do a take-off of a Munster standard where Marilyn, the normal-looking Munster, brings home a boyfriend who meets an array of monsters. In this case, the normal-looking one is Crystal, the only member of the Inhuman royal family who does not wear a mask or costume. She meets the Human Torch, mistakes him for an Inhuman, and takes him home to meet the family. Being the Marvel Universe, the family has hidden powers and wears masked costumes instead of being monsters. Things go as well as can be expected until Crystal introduces the Torch to her sister - Medusa.
This tied nicely into prior stories. They never explained where Medusa came from and there was some speculation that Medusa had escaped the Torch a few issues earlier because he was attracted to her. She was a bit old for the Torch but her younger sister...
I haven't looked it up yet but I think that there was even a reference to the Munsters in one of these issues.
The Torch was involved in a few 1960s situations. Back when he had his own strip, his girl friend, Dorris Evans, hated his powers. While dating a superhero might seem glamorous, their dates were constantly interrupted. Dorris wanted a regular boyfriend instead of one who burst into flame. This echoed TV shows of the time like Bewitched where mortal Darrin wanted his wife Samantha to stop using her supernatural powers and live like an ordinary housewife.
By the end of the story arch that introduced the Inhumans, Johnny and Crystal were deeply in love but were separated. The separated, anguished lovers was another 1960s staple (it happened constantly on Dark Shadows). While Reed and Sue managed to get married, most other superhero had some reason for not being able to tell his true love about his feelings.
Doomed Superhero Romances of the 1960s (off topic but a fun exercise)
Spider-Man/Betty Brant - hated violence after her brother's death
Iron Man/Pepper Potts - not fair to date with a heart condition
Daredevil/Karen White - not fair to ask someone to date a blind man
Cyclopes/Marvel Girl - not fair to ask someone to date a man whose eyes can destroy buildings
Hulk/Betty Ross - do I have to elaborate?
Thor/Jane Foster - romance forbidden by Odin
Human Torch/Crystal - Crystal imprisoned in a sphere of negative energy
Silver Surfer/Shala-Bal - separated by Galactus
And a few DC late-entries:
Metamorpho/Sapphire Stagg - not fair to ask her to date a freak
Dead Man/Lorna Hill - he was dead