Keeler doesn't seem the type to have premarital sex with someone she recently met. This was 1930 after all and many (perhaps most) women then were not likely to do so for social and or religious reasons. The 1983 book Yesterday's Child by A.C. Crispin does have Spock siring a son with Zarabeth back in the remote past on the planet Sarpeidon.
Premarital sex has occurred throughout human history, though the legal and social penalties for it vary greatly, from a shrug and "so what" to burying the woman alive (as the Romans did to any Vestal Virgin caught having sex - whether or not she had consented or had been raped).
What Spock and Zarabeth did is irrelevant to what Edith and Kirk might have done. The idea that Spock would "regress" 5000 years to the point of liking meat was silly. It did, however, give us a time frame for Surak's reforms when Vulcans embraced peace (that apparently included becoming vegetarians). It also made it clear that Vulcans are perfectly capable of sex outside the pon farr cycle. As for Zarabeth's cultural norms, we don't know what they were. Zarabeth came from a high-ranking family and her exile in the past was politically-motivated, not because of the impending nova.
In the era during which the story was written, I believe the prevailing and generally-accepted view was that time moved in a single line, and in one direction only.
If, somehow, someone were able to travel to an earlier point on that timeline, they could theoretically alter anything / everything from that point onward. All time-travel stories from the Original Series followed this model -- you travel into past; you break something; you must fix it in order to restore the "past" and return to the "present".
While the idea of a multiverse with multiple / alternate timelines
had been proposed on a number of occasions previously, it would not gain wider acceptance in either fiction or science until quite some time after the Original Series ceased production.
Robert Silverberg's time travel novel
Up the Line was published in 1969 (about a group of people who are Time Couriers, escorting tourists on one or two-week tours into the past; of course shenanigans and accidents happen, and they end up having to deal with alternate timeline problems). He mentioned in his email group that the novel has been optioned (I'd love to see an adaptation of that book)... though there are some aspects that the readers of 1969 wouldn't have minded so much but very definitely don't sit well with modern readers (there are some racist and sexist elements in it that would need to either be changed or deleted).
Greg Cox was a co-author of one of the novels based on
Up the Line (I assume Silverberg authorized other authors exploring his setting). It's a good little adventure story.
Poul Anderson wrote a terrific series of Time Patrol stories that initially appeared separately, but were later gathered together in either novel or anthology form. The one I've noticed being most frequently included in other anthologies is "Delenda Est" - originally published in 1955.
One thing I noticed about Anderson's stories is that the characters grapple with the ethics of changing history. Yes, they are tasked by the Danellians (far-in-the-future species that humans will someday evolve into) to keep history on track so they will exist. But the real human price of doing this makes it hard, as they ask themselves if they have the moral right to snuff out the lives of billions of innocent people just for being born in the "wrong" timeline (what makes any timeline "right" or "wrong"?).
Yeah, that appears to be the time-travel mechanism (a single timeline that you can change and restore) for "The City on the Edge of Forever". In "The Naked Time" it also appears to be able change their fate. "Tomorrow is Yesterday" and "Assignment Earth" doesn't tell us whether Starfleet winks out of existence or not when the Enterprise time-travels back to Earth in the 60s but does suggest the crew was worried about screwing up the timeline.
"All Our Yesterdays" does give us an interesting variation on time-travel. Beta Niobe sent it's entire population into its own past without affecting the escape building which I think is remarkable. It didn't seem to have any temporal protections like the Guardian yet no one sent into the past apparently altered their timeline (unless Atoz is an example of multiple timelines in action at once?)
As far as a multiverse we do have the alternate universes / alternate timelines of the antimatter universe in "The Alternate Factor" and the evil timeline or universe in "Mirror, Mirror" but yeah TOS doesn't have any examples where every decision spawned it's own universe that they could visit (we don't see that until TNG's "Parallels").
I think despite the era TOS was made they still had an amazing variety of time-travel and multiverse ideas.
It's been theorized that part of the "preparation" includes a mental block on doing anything that would change history. Remember how the official acted in the time setting Kirk was in, when he was accused of witchcraft. He kept denying that he came from the future, wanted nothing to do with it. He was actively afraid, which suggests that there might be some rather painful consequences for anyone who tried changing history.
Yeah the Atavachron prepared "cell structure and brain patterns to make life natural [in the past]" but there was nothing about the sudden appearance of additional people in the past or warnings to not alter history especially since they were allowed to go "wherever they wanted to go." There were surprisingly no safeguards to their timeline. I do wonder if the thousands of "verism tapes" they are able to travel back to are portals to different timelines in their past where they cannot alter the timeline they are on though.
Or a more darker possibility is that they are transformed into data and added to the verism tape simulation of the past and when the star went nova that was it for them.
That's a grim idea. But as we see, Kirk, Spock, and McCoy interacted in some very physical ways with the people of the past, whether getting hurt, having food to eat, and in Spock's case, having sex (whether or not you consider it valid that the result was Zarabeth's pregnancy in Ann Crispin's novels).
I'm aware of that . However, as presented in the episode, via music, lighting, etc. Keeler seems such a saintly, near angelic personality that I'm inclined to believe she is one of the more chaste types. Of course, being a fictional character there is no way of knowing one way or the other, of course.
The original script portrayed Edith as a "Salvation Army Sister" kind of person who ran a mission for the homeless, to make sure they would have at least one meal a day and offer whatever other help she could.
The difference, of course, is that Edith didn't preach from the bible. She made speeches about the future she hoped would come to pass, in an attempt to motivate the men there to not give up.
