As for why... well, think of it this way. If there's the threat of persecution (of any sort), are you more likely to admit to being what is being persecuted when you're alone, or if there are others with you?
In general, people would rather suffer with others than suffer alone, it's human nature to be a ****ing chicken **** until you meet someone else like yourself. Because humans are cowardly and never stand alone, they can only ever act with the support of others. Even if you actually have to courage to face persecution all on your own, you'll never be heard - at best you'll be shrugged off. At worst you'll get **** for it. But the more and more people there are, the more likely people will be forced to hear you. Squeaky wheel gets the oil after all. Which is to say the longer you ****, moan, and complain about things increases the likelihood you'll get what you want - but it further increases with the more people there are.
And now the question is to why that is. First, it's ****ing annoying so people generally have to listen. Second, they give you what you want just to shut you the **** up. Third, you're drawing attention to an issue to people never have really thought about before and you're now making them think about it. Fourth, depending on how you go about this this can either be a motivator for people, helping them to understand better, or will seriously piss people off because to them it seems like a bunch of ****ing and raising Hell for no reason. Fifth, because the more people there are the more likely people not involved with it are to come across it.
That's basically the gist of it. Now, as for how these things start. Organization, that's how. Once upon a time, information was more carefully spread via word of mouth and through preexisting social circles or societies. It's the ebb and flow of information. Basically, people really have always liked to gossip. And just like now, back then there were people who thought "well, what's wrong with that?" and all it takes is to act upon one idea and to find another willing to do the same (if it requires another person).
That's how anything and everything spreads, the ebb and flow of information, ideas, all cycling back around until you reach the point where things have to revert.
Which is called a cycle of rebellion. Eventually, generations rebel to the point where everything starts all over. Eventually society will reach the point where a norm (just a norm) will be settled upon and anything else will be frowned upon. This norm can vary from place to place, since there are culture differences and whatnot. Let's take this as an example, cousin marriage (just to be different from the typical example of homosexuality here, lol).
Once upon a time, this wasn't necessarily frowned upon and was considered commonplace for royal families as a whole (most notable being the British royal family). For the everyday common-folk though, it wasn't quite as common. Not unheard of, but not near as common because it was more beneficial to most families to make connections with other families via marriages. So basically, unlike royal families, they didn't have a reason to do this but this doesn't necessarily mean it was frowned upon. Contrary to the ever-popular belief that women were nothing but breathing sex dolls used to churn out babies as though that was their only function, women actually typically got a say in the matter as to who they married. Common women, of course. Typically when we never get tales of the lives of common people, we just hear about how this royal woman and that royal women were in arranged marriages and we just began to associate that lack of freedom with that generic time period.
Moving along, so basically it didn't used to be so frowned upon... then, as much as it pains me to say this, America ****ed and raised Hell and went full on revolution (as opposed to rebellion) on the British Empire and, because this is necessary during any war or revolution, they created propaganda. Now like I said, common people as a whole never had a reason (political, or whatever) to marry their cousins. So once the vast amounts of propaganda started churning about, the common folk in America (self-admitted Know-Nothings, I **** you not it was a ****ing party) just believed whatever they got told based on the little evidence they were presented. Which, by the way, was Hemophilia. This is just what started the whole thing though, in America's attempt to further alienate ourselves from the British Empire by making the royal family look like a bunch of inbred ****tards.
Of course, mud-slinging as a whole was much more common and vicious back then so... yeah...
But this is where it started, in America's attempt to detach ourselves from the British Empire by means of propaganda. This wasn't the only way, obviously, just one of many.
Moving past that point, that notion just sort of stayed with people, and because of the ebb and flow of information if it was a subject that came up peoples minds went to 'nastybadwrong' even if they hadn't actually ever been taught that is was wrong before. By virtue of being unheard of to them, unfamiliar in concept, therefore not common it's rationalized as something that's wrong... because it's not common, ergo not normal. And to further cause problems for it, if got around you had married your cousin and you had a deformed child then that became the association. And much like all information, this notion spread and it varied from place to place. Leading to absurd notions about children with 3 heads, webbed feet, etc. and people actually believing it.
Which can scare the ignorant masses, you know.
Moving along to this era, it's still frowned upon but because of the internet the ebb and flow of information is more accessible, more available, faster, slightly less prone to the telephone effect (as in the rumor changes with the more people it passes through). Facts are more open and accessible as well. But because people like to believe they're right, even in the face of evidence, they always fall back on the argument of it being 'nasty' like that's a good enough reason or something.
But look at laws and social attitudes to the issue now. Interestingly enough, although from what I gather British society as a whole frowns upon it, it's still legal in the UK. As a matter of fact, the US is the only western country which has any ban on cousin marriages at all.
Because it's left over residue from propaganda, myths, and notions that should have long sense been revised. Although in the US some states have revised them, but because the social norms still persist, because there's not enough people willing to rally behind the notion of it being acceptable, it gets brushed off and treated with disrespect.
Even if the people are making the exact same arguments as another group. For example, people say "love is love"... up until it involves something they find nasty, bad, or wrong. In which I compare it to homosexuality and the argument for Same-Sex Marriage. The notion is love is love, yes? And we should stay out of other peoples bedrooms, yes? Well, I once had this talk with a guy online about how he got to be involved with his male cousin. He found it odd he got less **** over them both being guys and more **** over them being cousins, even though the one major argument against the notion of cousin marriage (or sexual relations) had been totally removed from the equation. And that is the issue of reproduction. One major argument against cousin marriage is having deformed children, but the factor is removed in this case, so... why is it still just as wrong if they can't have children biologically with each other?
There is the hypocrisy. It's not about reproduction, it's about personal bias, prejudice, and what society believes to be wrong. Even if the major factor arguing against it is removed, the 'gross' factor persists.
So what this all leads up to is this... things change because people adapt and rebel. Make enough noise, people eventually get used to whatever notion you're making noise for. Otherwise, even if you're making the same argument but for another issue (and it applies to both issues), you're written off and treated as a freak, an anomaly, not common enough, not enough of you around to actually worry about. Because society has to adapt to social threats, either by integrating it - if the threat is big enough. Or by squashing it - if it's small enough.
All in all, things change because people get fed up, they **** and raise Hell and depending on how much Hell they raise - society is forced to accept it as a norm, or they risk social revolution. If things are not just accepted, not taken seriously, when they should be then a social revolution will happen. If the threat is minor then it can be silenced.
Now, once something becomes a norm, expect more people to be open to the notion of it as a whole for one reason or another, esp. the younger generations having grown up with it settled in as a norm.
Also, because you see people like ****ing and raising Hell over something. People like feeling like they're oppressed because they keep something secret, and enjoy the feeling of freedom that comes with shouting to the Heavens who they ****, how they **** them, when, and where. This mostly happens when people feel like they're alone in the matter, then find out they're not, then figure out they can't be the only ones around like that.
So people just like ****ing about ****, esp. now that they can do so more easily and it having become commonplace.
But that's how things change. It all starts with a small spark, then not long later you have a wildfire on your hands. And the organization part has only increased with the internet, which is why ****ing and complaining about things is more commonplace now - the internet has made it easier.
As for sexuality itself, people want to seem open and accepting, esp. women, so often Heteroflexible women (some men but it's mostly women) claim to be Bisexual. Because some people think rejecting the notion of Homosexuality, as far as they're concerned, that they'll come off as being homophobic or something. That and Bisexuality in women is considered 'hot' so it's more socially acceptable for women to be that, as opposed to men because 'men can't be Bisexual' according to some people.
Point being the more open something is, the more open people are to being that. The opposite is also true. Because, yes, society does have a large say in how people end up sexually. Social factors are always important in sexual development. Social factors are always important regardless.
One other thing, one social movement can spark another.
