It's really very simple. The nation is ruled by the reliance of "gay rights" on two propositions that have been accepted by the vast majority of people:
1) Sex is not about creating a new life, and, in fact, if two people engaging in sex happen, by some unforeseen series of events, to create a new life, they are to be freed of consequences by whatever means necessary. The corollary - which is that since the purpose of sex is sterile, the receptacle is irrelevant - follows.
2) The marital relation exists solely for one's personal happiness, and, if such happiness should disappear, even temporarily, the marital relation becomes "dead" and you are to be freed of it, by whatever means necessary.
These two (false) propositions were accepted, overwhelmingly, by 1975 at the latest.
After that, it just took a few decades of propaganda to get America's strong pro-fornication majority past the icky bits about our small homosexual minority and to allow them into the sexually liberated world created by heterosexuals for themselves.
You nailed it. Heterosexual immorality is yawned at. Living together before marriage is something considered normal now. THAT is where the dam broke. Yesterday was just another step down the same path we have been on for decades.