Or just a play-date. Since we know their parents hung out together it's not really unusual, what's unusual is that apparently neither Doc or the Monarch remember it, especially since Doc's memory of his childhood is generally very sharp; but he clearly doesn't remember everything.
The Blue Morpho and Jonas Snr stuff is kind of interesting because of that scene in the Morpho Cave with the old video. It's Jill St John and Stella Stevens that kind of give it away: they were both active much earlier than video tape was commercially available (Stella Stevens was born in 1938 and a Playmate in 1960; Jill St John was ten years younger but worked as a child actress). They never really worked together on anything major so there's probably a sweet spot (in the writer's minds) when they were both at their hottest, and that's when the scene is set.
However, the actual reference of the scene is to Bob Crane, who liked to videotape his sexual encounters and did so with help from his friend John Henry Carpenter (who was later suspected of murdering him).
Crane and Carpenter met during the run of Hogan's Heroes (which ended in 1971) and continued their arrangement right up until the year of Crane's death, 1978. This was several years before the release of the first consumer video cameras (camcorders), so Crane was necessarily using pro-grade equipment, which had been available (and increasing in quality/decreasing in size) since 1951.
So that one scene likely sets itself anywhere in the 1970s, but possibly toward the early part of the decade (and we know Jonas doesn't "die" until after the release of Sharky's Machine in 1981) because of the women involved.