if someone allows it to be a topic of discussion, and says no, that's a lot of the way to them giving in.
there are other social rules where you're not supposed to do something, and a lot of the social mechanism to stop you involves refusing to think about or discuss the issue in the first place.
once the ice is broken, once there is a foot in the door, it's much harder for ppl to resist the social sin. they don't have actual arguments to resist with, and the social rules don't give them much help cuz they focus on blocking things off at the start without considering the issue.
it's sorta like if you say something taboo, then if you aren't immediately shut down (e.g. told to STFU) then just having said it makes it less taboo. even if you're just like "i wonder if the taboo about X is a good idea" and then you consider some args on both sides and end up concluding it's a rational taboo, that still kinda messes up the social prohibition that makes it a taboo.
an example is sending an elderly parent to an old folks home. that's not exactly taboo but there's pressure not to do it or even consider it. it's hard to bring up. it could offend ppl. when you say it, ppl might react immediately negatively, like "oh we could never do that". if no one reacts immediately negatively, then it's just become socially acceptable to consider it, and that's already a bunch of the way to doing it (even if u merely wanted to consider it, u've now helped it happen, esp if you don't have all the relevant decision making authority).
or consider ppl who are shy about sex. if someone asks "do you want to have sex?" and the other person isn't immediately offended, and is actually willing to discuss the topic, then they're already a lot of the way to having sex. breaking the ice is one of the hardest parts – or in other words lots of the pressures are front-loaded.
or it's similar with sexual fetishes. if you ask your spouse to do one of those, then the way it works socially is s/he has to be like "no way" immediately without thinking about it. otherwise it gets significantly harder to resist and say "no". even if they think about it and discuss and say "no", now that the topic has brought up you can just keep asking and giving reasons or whatever and wear down their resistance. their resistance isn't as effective once past the initial reaction.
or doing an intervention for someone. that's really awkward. and if you suggest it, ppl might say immediately "no, that's too drastic and mean" and shut down the idea. but if no one shuts it down then it's become socially acceptable to your little group of friends who were talking, and it's got a good shot of happening.
or committing someone to a mental hospital against their will. this might be a thing some relatives are considering but no one wants to say. and if someone says it outloud too early, ppl will shoot it down like "no way". but if someone manages to suggest merely considering it, without getting the idea immediately shot down, then they are a bunch of the way to actually doing it.
another example: pulling the plug on a spouse in a coma at the hospital.
one that is NOT an example is atheism. maybe it was in the past? (or is now in some countries like Iran?) but now questioning God's existence in the West is so well known and socially acceptable that allowing it to be a discussion is not dangerous to God believers. believers have developed knowledge of how to deal with challenges from non believers. they don't just rely on avoiding the discussion or maintaining some sort of taboo.
or both capitalists and socialists can socially-safely treat the other side as legitimate to discuss. they don't rely on just refusing to discuss. they're used to debate and don't consider the other side's ideas taboo. their resistance to switching sides is not front-loaded. breaking the ice like mentioning capitalism could be false doesn't really matter.
environmentalism, like recycling or global warming, is more front-loaded. they try to shut up debate more than socialists or capitalists, though not entirely. there's a lot of effort currently going into trying to make environmental skepticism an unthinkable taboo.
what are other examples and non-examples?
btw this stuff doesn't just affect discussions outloud with other people. it works fairly similarly with self-discussion. like if the example is an affair, just making pro and con lists in your own head is damaging to your ability to say no to the affair. it stops feeling like a taboo or illegitimate non-option, and starts becoming more possible to think about, discuss, maybe even do. or like consider if you've made pro/con lists in your head and then the other person suggests an affair. now it's harder for you to be like "wtf? no way! don't ever ask that again. how dare you?" cuz if you say that you're lying. and the person might guess that (or just hope it on general principles – nothing to lose for trying this tactic even if mistaken) and be like "you've thought about it, i can tell, don't pretend this is just my own deviant idea that never crossed your mind". if that's true, it's much harder to just be like "omg you're a deviant, what a bad idea" and block discussion entirely.
The Fountainhead illustrates the affair example:
“Your wife has a lovely body, Mr. Keating. Her shoulders are too thin, but admirably in scale with the rest of her. Her legs are too long, but that gives her the elegance of line you’ll find in a good yacht. Her breasts are beautiful, don’t you think?”Merely allowing a discussion of the topic is a large social concession. Slapping is the kind of action which can shut this down, socially.
“Architecture is a crude profession, Mr. Wynand,” Keating tried to laugh. “It doesn’t prepare one for the superior sort of sophistication.”
“You don’t understand me, Mr. Keating?”
“If I didn’t know you were a perfect gentleman, I might misunderstand it, but you can’t fool me.”
“That is just what I am trying not to do.”
“I appreciate compliments, Mr. Wynand, but I’m not conceited enough to think that we must talk about my wife.”
“Why not, Mr. Keating? It is considered good form to talk of the things one has—or will have—in common.”
“Mr. Wynand, I ... I don’t understand.”
“Shall I be more explicit?”
“No? Shall we drop the subject of Stoneridge?”
“Oh, let’s talk about Stoneridge! I ...”
“But we are, Mr. Keating.”
Keating looked at the room about them. He thought that things like this could not be done in such a place; the fastidious magnificence made it monstrous; he wished it were a dank cellar. He thought: blood on paving stones—all right, but not blood on a drawing-room rug....
“Now I know this is a joke, Mr. Wynand,” he said.
“It is my turn to admire your sense of humor, Mr. Keating.”
“Things like ... like this aren’t being done ...”
“That’s not what you mean at all, Mr. Keating. You mean, they’re being done all the time, but not talked about.”
“I didn’t think ...”
“You thought it before you came here. You didn’t mind. I grant you I’m behaving abominably. I’m breaking all the rules of charity. It’s extremely cruel to be honest.”
“Please, Mr. Wynand, let’s ... drop it. I don’t know what ... I’m supposed to do.”
“That’s simple. You’re supposed to slap my face.” Keating giggled. “You were supposed to do that several minutes ago.”
If the discussion were more rational, with serious arguments, it wouldn't change the social meaning of being willing to consider the topic in a discussion.
merely treating a topic as discussable has social meaning.
the social rules block paths forward. you can choose: block the discussion or defy the social rules.
you may doubt that affair discussion is an important path forward, b/c you have a low opinion of affairs. but i bet you have a higher opinion of something else which involves similar social dynamics.
also even if no affair is ever a good idea (past or future), discussion of affairs would still be an important path forward. because sometimes people want to have affairs, or think it's a good idea. even if they are always wrong, discussing it is still good. they could learn they are wrong and why, and then be happy to not do the affair. (but the social game rules are incompatible with this approach.)
one question is: how do you have such discussions without the social meaning? if you just want to talk/think about it but not change the social landscape. can you? it could be impossible since social rules are flawed, so they may not be compatible with this; the only solution might involve rejecting some social rules stuff. but maybe there's other solutions. post your thoughts in the comments!
on tangent, what do you think of Ayn Rand's affair? one notable thing is it wasn't secret. on TV affairs are usually secrets. (one reason is if you ask your spouse if having an affair is OK, that's one of those things where the socially acceptable response is to freak out and immediately shut down discussion)
Rand's affair was secret from the public, but not from her husband. I wonder how common non-secret affairs actually are in real life. Or affairs where a spouse knows about it without being told but doesn't say anything.
another issue is Rand was mistaken about how good a thinker Nathaniel Brandon was. is that a coincidence? did lust play some role in this mistake? or was she not mistaken at the time, and he changed later? (btw just merely raising the possibility that lust played a role in the mistake – not a nice thing to consider – has social meaning. it's harder to bring up that unkind possibility initially than it is to discuss it afterwards. the resistance is somewhat front-loaded.)