I highly doubt actual sexual orientation has shifted. Its just that its the hot fashion to be a ‘cultural’ LGBT these days and women are far more open to that sort of thing.
That seems to be the case with the ones I know claiming it.
It’s peer pressure and socially acceptable. They get a lot of approval for it.
This is one part of the problem. The other major contributing factor is pornography. The porn habits of young men these days are causing them to request more perverse activities and have unreal and disgusting expectations of and behavior toward young women these days.
When your choices are mostly soibois, incels and 4chan assholes it becomes hard to maintain hope that there are any chivalrous men left in the world.