patricius79, This is not new. There have been different variations of sexual perversions by those who claim to follow Christ, and some of them have tried to make it official teaching in their man made churches.
You are asking about fornication, but this also applies to polygamy as well. The same argument is used Jesus said not to divorce your spouse. People then say well he did say no sex with animals, or with women and men before marriage, or with 6’1" women, or with blah blah blah. It is the Catholic Church established by Jesus himself Mt 16:18 that offically taught what Jesus meant, and the apostles conveyed that truth in scripture passages such as not allowing the marriage bed to be defiled. Of course with private interpretation anyone can come along and say this passage doesn’t mean or apply to fornication. Even though fornication is listed multiple times as a deadly sin, and the early Church fathers spoke of its evil often.
The most recent scandals of this have come through the protestant reformation. In U.S. history you have the Mormons, when applying for Utah’s statehood" sent delegates to a protestant congress to remind them that Martin Luther himself allowed polygamy by granting Phillip of Hesse multiple wives as Jesus never said ONLY ONE wife. And as we should all know from the words of Martin of Luther, “no where in scripture does Jesus forbid more than one wife”. This is also the one an only Father of protestant religion who stated if the wife is unwilling let the maid come.
At the end Jesus promised the Holy Spirit to lead the Church into all truth, and Paul proclaims it the pillar of truth. The Church is protected by the Holy Spirit not to teach error. The bible alone crowd make use of Gods word sometimes to call evil good and good evil.