On the show:
Pat Matthews had an illegal abortion (offscreen) and was told she'd never be able to carry another baby. She married John Randolph and they had a "miracle pregnancy" several years later, where she gave birth to twins Michael and Marianne. In true soap fashion, the twins were SORASed to late teens within several years, and were sent off to college due to lack of storyline.
Harding Lemay decided Michael was a popular enough character to endure the slings and arrows that would come their way by writing him as gay in 1976. M&M were the children of popular characters (Pat had been there from Day One). As he said, it's easier to write such a story with well-known/liked characters, but he should have added that it's also harder for the network or sponsor to order the show to kill off or write out a popular character involved in a story they want to end. Anyway, Marianne returned to town first and kept making vague remarks to others that she and Michael were keeping some secret from their parents, that Marianne was worried that their parents might look at them differently once things got out in the open, etc. These are the "things in the outlines" Lemay was referring to, things that laid the groundwork for the revelation that Michael and his (male) college roommate were in love. Marianne obviously knew what was going on, but was letting Michael tell their parents.
Lemay was not writing Michael's coming out just to say
Hey, look how progressive we are or to claim any sort of "Soap Opera First". He had decided that John and Pat Randolph were too boring and wanted to break the couple up for a few years. He just could not come up with a strong (that is, believable) reason for them to break up, since they were so solid and didn't have many explorable flaws. In this way, the twins were just plot devices for the adults. They would visit, Michael would drop his bombshell, and after some dramatic scenes, Michael and Marianne would return to school, leaving their parents alone to argue and ultimately divorce as John would be unable to accept his son's being gay, while Pat would be more understanding (since those kids were a gift she never expected to receive).
No one really knows who punted the story (P&G or NBC), but since the setting-up had already aired and the plot outlines of John and Pat's divorce were already written and submitted to the network, Lemay had no choice but to come up with another "excuse" (plot point) for John and Pat to divorce. I'm sure Lemay was angrier about having to do a hasty rewrite than he was about not getting to write "a coming-out story" because the ultimate "secret" Marianne had been alluding to was not about Michael at all: the rewrite was that Marianne had had a secret abortion (legal, this time), a plot point Lemay felt wasn't as strong but the best they could do on short notice.
The Paley Center has a copy of an episode from that period, with a description on its site:
https://www.paleycenter.org/collection/item/?q=bell&p=57&item=T:03100