So-called "conservatives" say this because the LGBT mafia weaponized their own compassion and politeness against them, we've increasingly become a secular nation, they've bought into the myth of gays being harmless, and it is political suicide to oppose it, in that order.
There's several things to unpack with the LGBT phenomena that I don't think I could fit it in a single post without it being autistic as fuck and sperging, so I'll try to keep it brief.
To the first point, weaponizing compassion. This has long been a tactic of the left in general, but it has been particularly egregious with the LGBT cult. America - and by extension most western nations - has long considered itself to be a civilized culture. We have rule of law, a justice system that promotes fair treatment and trial, we believe men are innocent until proven guilty, we have laws against cruel or unusual punishment, and the list goes on. Some nations will take someone accused of stealing and just chop his hand off right then and there. Some nations will cane you publicly for littering. Some nations will take a gay person and just throw them off a roof, without any trial whatsoever.
We, however, see ourselves as above these "barbaric" policies. We treat criminals with respect and dignity, and the LGBT cult has weaponized this against conservatives. You shouldn't be so mean to gays! After all, even if it is a crime, who was hurt by it? It may be unnatural, but it's a non-violent offense, and you shouldn't be so cruel to them!
This led directly to the second point, the secularism. We, being such an enlightened, civilized nature, slowly began to slip away from the idea of God and objective morality. We trusted in science and studies and the Worldly-Wisemen in our midst who insisted that our policy should be based on pragmatism and reason. Now, this in and of itself is not objectionable, we honestly should base our policies on real-world studies and logic, but this is another tool the LGBT crowd likes to use frequently: citing a lack of evidence for a subject which has not been studied, and then using that lack of evidence as justification for whatever foul degeneracy they propose. They'll say "well prove to me that this is bad!" while not realizing that, one, the evidence you ask for does not exist because nobody has ever studied this before, and two, they are the ones making the assertion, therefore the burden of proof lies with them to prove their ways are good and harmless, not on us to prove that they aren't.
This led to the third point. With no prevailing evidence of being gay causing any major societal harm, it simply caught on that gays just wanted to be left alone. It became seen as cruel and heartless to tell people "no" to something they supposedly couldn't control (more on that later). They weren't hurting anybody, there's no proof, so why the hell not?
Over time, over many years of slowly seeping and marinating into the society, pushing the boundaries ever further, it has become so entrenched in our society and we've fully bought into the myth, that we now reach the fourth point: it is political suicide to oppose it. LGBT has gained such a stranglehold on our entire society, that even the slightest bit of pushback is seen as draconian and barbaric and bigoted. Now, even when we do have studies to show the downsides of this lifestyle (higher rates of domestic violence, higher rates of STDs, higher rates of mental issues, tendency towards promiscuity, strong correlation with childhood abuse, etc...) you could never get these studies publicized or disseminated, as nobody would dare say a word against the LGBT mafia. They successfully won over the culture. Nobody dare speak against them, lest they be branded a traitor and a bigot, and their opponents, still living in fear, a false sense of compassion, and having bought into the myths, will never take a stand against it.
My opinion on LGBT has always been the same. If you want to put your dick in another man's ass, or you want to rub coochies with another lady, fuckin' go for it. You obviously can't go door-to-door, breaking into people's homes and shouting "THERE BETTER NOT BE ANY GAY SHIT IN HERE!" That would be impractical, to say the least. However, that's about all the freedom they should get. You can boink who you want in private, but that's it. You will not be celebrated, you will not be promoted, you will not be officially recognized by the state, you will not be married, and you will not proselytize the ways of LGBT to the common man. You will keep that shit to yourself, the same with every other sexual fetish, and you will leave the rest of civilized, polite society alone.